Dream Theater:

This is the band's official page. It is actually a very great page. It has TONS of tabs. You can read on how to join their fanclub, and also their email mailing list here. This and Mike's page (the next link) are the best places for news.

www.mikeportnoy.com - This is the official webpage of Dream Theater's drummer. There are some MP3s and video files here. It has a very active message board.
dreamtheater.mit.edu - This site contains TONS of MP3s. There are not only a lot of rarities, but also a few complete shows, and some LTE stuff too.
http://www.johnpetrucci.com/ - This is the offical webpage of DT's guitar player. There are a few MP3s, including some of the solos from SFAM (and all you hear is the guitar, nothing else.)

Tablature

http://www.harmonycentral.com/Guitar/tab.html - This page has a great interface for the OLGA.