Regular Observer:  You will just be a regular observer, like always.  If this happens, choose a team and a class that you want to cheat with, then reconnect and go back to step 1 and try again.

Floating Observer:  You will be like a regular observer, only hovering in mid-air.  If this happens, choose a team and a class that you want to cheat with.  You're player will be able to hover.  This is identical to the Flying cheat, described in another section.  If you want to go into Ghost Mode, however, you must then reconnect and go to step 1 and try again.

Observing Class:  This is what you want to be.  Sometimes you will be the last class you choose, sometimes you will be a random class.  Proceed to step 4.

Floating Observer Class: You can use this to do the Ghost Mode cheat also, and it makes for a different experience, although it is harder to navigate.  Gravity doesn't affect you in this mode.  If you want to use this mode, go step 4, if not, reconnect and go to step 1 and try again.

Don't be alarmed if you have to reconnect many, many times.  Eventually it will work.  This happens to me often.
 

This cheat gives you the best of both observer mode and regular mode.  Invincibilty originates from the observer's side of the family.  Like an observer, rockets, grenades, nails, bullet spray, and people will go right through you, and will not do any damage to you in any case.
 

As an observer, you can fly by holding attack.  As a ghost, you will both fly and fire your weapon by pushing attack.  This can be combersome when trying to attack, because you are constantly being pulled when you fire, but it helps to be able to fly anywhere.  I suggest using the axe when flying, so you don't waste ammo.  If you did this cheat as a Floating Observer Class (step 3 above) then you will always be floating, and holding attack is the best way to move around.  One downside of this is that the Heavy Weapons Guy, who must be firmly on the ground, standing still, cannot use this 

cheat because the act of his firing causes him to move, and his gun won't shoot. Nuts.
 

You can kill anybody in the game, from any team.  No one is safe.
 

This is perhaps the most unexplainable aspect of the cheat.  For some reason, whenever you kill someone, it says they were killed by a person with no name, instead of the name you're using.  ie: QuakeVictim rides  's rocket
When you speak, however, it uses your regular name.  Coupled with the next feature, 

this makes you virtually undetectable.
 

As an observer, the server doesn't give a damn about what color or team you are.  This makes it very easy to change your color and team to match everyone else so you will be undetectable, or you can stand out in the crowd by making your name something odd, such as Team Anus.
 

Ghosts tend to pick up disease quite easily, and can spread it throughout the server in little time.  Or, if you are a medic, you can do the damage yourself.  This is the same as the observer infection cheat described in another section.
 

You cannot pick up ammo boxes as a ghost, but don't despair!  The bumbling Team Fortress creators have left a door open for us!  You can raid dispensers from either team just as you would use dispensers in normal Team Fortress, and quite easily I might add.  However, when you use a dispenser, it shows "Enemy's are using your dispenser!" on the screen of the engineer who created it.  They usually blow it up at this point, so be sure to get the goods fast.  Of course, the explosion won't do anything to you.  This is the only way to resupply your ammo, so you do end up using your basic weapon (the axe) a lot.
 

An interesting side effect to this cheat is the Shadow People.  These are people who, for some perplexing reason, have lost their skin, and are completely black.  It seems to be just random people throughout the server, and it makes it a little difficult to spot them.  I'm reminded of a misconcieved episode of Nickelodeon's Are You Afraid of the Dark.
 

Another odd effect of being a ghost is that sometimes, doors and elevators areinvisible. I'm not sure why this is, but it's sometimes helpful when you want to seewho's in a room.  It's a little frustrating when you first realize it, because you're trying to walk through a door and you keep getting pulled back through.
 

The frag list will start out blank in ghost mode, but people will appear when they get fragged or frag someone.

Ghost Mode is probably the most fun that I've had with a cheat.  It screws up the game more than any before.  It can easily turn the tides of any game with just one person cheating, let alone two or three or more.  After releasing this cheat on my page, I'm sure the whole world will soon learn of it, and eventually Team Fortress will get off their lazy asses to fix it.  So USE IT WHILE YOU STILL CAN!  Tell everyone you know, before it's gone!

There are so many odd aspects to this cheat.  Why does skins, typed at that exact moment, turn you into an observing class?   Why are the doors and elevators invisible?   Why are there shadow people?  Why do your kills register to a person without a name?  I'm willing to bet that there is more out there to this cheat.  Keep me posted guys.

When I first found the cheat (I discovered it myself, nobody told me), I realized that this what it, the promised land of cheating!  I knew that putting it on my page would eventually be the cause of its destruction, so if I wanted to use this as much as I could, it would be best for me to keep it all to myself.  But I remembered how much it sucked not knowing how people were cheating, and felt it absolutely neccessary to tell people.  I hope you appreciate what I go through for you. :)

If you are having problems with this cheat,  please re-read everything I wrote, and try it a few more times before emailing me.  I just know I'm gonna get spammed with questions about this.
 

Feign death by typing "feign" at console, or use the menus.
Turn AUTORUN on by pushing escape, choosing options, then selecting autorun
Roam freely about as a happy corpse.

Another way to do this is using speed cheating, described in another section.
 

Its harder to hit a corpse, especially an active corpse.  You can be invisible in shallow water.  You can slide under the red lasers in the Rock level and probably some other low-clearence crevices.
 

Feel a bit restricted as a sniper when your using autozoom and you can barely move?  Think the heavy weapons guy should be able to run as fast as a scout while firing his gun like a madman?  This cheat is for you.
 

First, you must have the Kick and Color Protection.  This allows you to cheat on servers that do speed checking (virtually all of them now).  Then you need to make your keys turn on speeding whenever you move, because the server will continuously change it back.  For example, bind uparrow "+forward; cl_forwardspeed 500"   There is a file in the Kick and Color Protection that does all these binds, it is called hwguy.cfg.  It will work for any class, though, it's just very handy for the heavy weapons guy.

Gives you a little "edge" against your fellow players when you speed.
 

A spy that can shoot while disguised (Credits to Adam)

Disguise both your skin and color.
Begin to disguise your skin again.
Stop going undercover by pushing special (the key that brings up the disguise menu).
Now you're undercover and can shoot.
 

People don't except much damage when there own team starts shooting them.  Check out the Color Change cheat for a way for any class to disguise it's color on any server.
 

You can still be killed.
 

On pre-2.6 final beta servers:

Floating sentries are a bitch to hit with grenades. With the second method you can really get sentries up there.
 

Here's a cheat I'm sure you'll all enjoy- placing multiple detpacks..
 

First of all, you need the Kick and Color Protection file available below.  This keeps you from being kicked off the server while doing the cheat, and is the backbone of the entire thing.

To place multiple detpacks, you must be kicked from the server while setting a detpack.  To do this, either bind a key to something that will get you kicked, such as bind w "color 1", or setup speed cheating: bind uparrow "+forward; cl_forwardspeed 500".  Get yourself kicked while setting a detpack and then you'll have 3. Sorry about the misleading 6-pack title, but it sounds good.
 

Do you even NEED to ask?  Multiple detpacks are fun fun fun, especially when you don't have to hang around setting them when you use speed cheating.
 

What would you say if I told you it was possible, on the latest servers, to rack up frags at the rate of 2 per second?  You'd probably call me a filthy liar. But what you don't realize is that it really is true.  I have seen the promised land, and it is good.
 

Three people are needed to do this cheat.  One needs to be a blue medic, one needs to be a red medic, and the third needs to be a red or blue spy.  The entire trio meets in some out of the way place where they won't be bothered much.  Under the stairs by

the water entrances in 2fort4 are good, as is behind the towers in well6.

Once assembled, the group begins by having both medics hack on the spy with their bioweapons.  One will continuously infect him, and the other will continuously heal that infection.  For each infection that is healed, that medic will receive one frag.  This will build up very quickly.  So if you had a red medic, a blue medic, and a red spy, the red medic would receive tons of frags.  If the spy was blue, then the blue medic receives the frags.  I recommend stopping once your frags reach 69.

I find it rather interesting that no one has told me about this cheat, I figured it out on my own.  Whats more amazing is that it's survived to the 2.6 final final version.  It such a logical cheat, once you know that healing an infected person gives you a frag, it doesn't take an Einstein to figure out how to take advantage of that.
 

One of the awesome things about this cheat is that it works on any server that gives the medic a frag for healing someone.  I know that the 2.6 final final servers allow this. It's quite easy to get to the top of the frag list with this cheat, and could be helpful in clan battles.
 

You need three people to do this cheat, and sometimes its hard to get a little privacy in a level without someone (from either team, mind you) interrupting your frag party.

It may be difficult for some people to find three people, especially since recruiting people right on the server makes your conversation public, and then the whole server knows your plan and will undoubtedly take great pleasure in ruining it.  Its fairly easy for me to get three people, though, cause I have about a hundred other cheaters on my ICQ contact list and most of them would be glad to come cheat with me.

Only one of the three people get the frags, so you may want to switch characters occasionally to give everyone a fair shot.

The infecting medic has to be careful not to whack the spy to fast, or eventually he'll be killed.  However, if you get a second healing medic, then I bet you could all whack away at top speed and have two people racking up frags.

An alternative to this that makes it much safer is to use an infecting observer instead of an infecting medic.  Then you can do this cheat with only members of one team visible, and people won't kill you as often.  Check out the invisble outbreak section for how to become an infected observer
 

Any class can permanently change its color to the other teams for easy infiltration.  Now the spy's not the only one who can disguise.
 

Get the Kick and Color Protection below.  This allows you to change your color, and the server is unable to kick you off or change your color back (though it will continuously try).
 

Surely anyone can see the benefits of any class being able to change colors and use thier weapons as normal.
 

This adds the ability of flight to any class in Team Fortress.  It is actually the springboard for the Ghost Mode cheat.
 

When you are connecting to a server and you see the words "Checking Sounds...", enter the word "skins" at the console.  When the game starts up, several things could be 

happening.  You want to be a floating observer.  Keep reconnecting and typing skins until this is the case.  Once you are a floating observer, pick your team and class, and you will stay a floating character.
 

You can fly across gaps, float up to hidden away places, etc.
 

* Sniper crosses glow - lets you light up any dark place just by aiming there and makes other snipers visible.
* Neon skins - Every player will have bright white skins, perfect for identifying players in dark corners and at long distances, not to mention jogging at night.
* Detpacks and Sentries are gigantic - you won't be able to miss a detpack or sentry this size.
* People glow - You'll see people approaching by the way the wall starts glowing.
* Dead bodies glow - Each corpse is like your own personal flare. How sweet.
* Dead bodies rotate - This makes it easy to identify feigning spies as the corpse NOT

rotating.