Rebel Assault 1

At the opening screen (spinning gold guy) with the
joystick do this : UP & fire, DOWN & fire, LEFT &
fire , RIGHT & fire. You should hear a chime and
a spoken "LucasArts" during the logo.

It enables this cheat keys :
[1]-[9],[A]-[F] : Level select
[Esc] : Advance level or stage
[+] : Substract damage
[-] : add damage.
[Z] : restart game

We also have these level codes :

Easy Normal Hard
------- -------- -------
4 Falcon Biggs Ackbar
7 Anoat Kaiburr Fornax
11 Yuzzem Mynock Bespin
15 Brigia Dagobah Kessel
End Greedo Mimban Organa (Information from Cheat Machine)