ACE - Weaker shields
BLAMEUS - See pictures of the game designers
CHICKEN - Bonus level where you play as an AT-ST
CREDITS - View credits
DEADDACK - All Levels
DIRECTOR - View intermission sequences (new option in Settings menu)
FARMBOY - Fly Millenium Falcon
HARDROCK - Changes Luke's face on title screen
IGIVEUP - Unlimited Lives
KOELSCH - Fly a car (Electra) instead of a V-Wing
MAESTRO - Music Test
RADAR - Enhances radar. Ships flying higher are brighter, ships flying lower are darker
TIEDUP - Fly a Tie Interceptor (push up from Millenium Falcon)
TOUGHGUY - All powerups

Enable Naboo fighter from Episode 1:
Enter the password HALIFAX?. Then select 'enter code' and type in !YNGWIE!. Select 'enter code' again to enable the Naboo craft. To disable, enter HALIFAX? then any other word.
Bonus Levels:

Battle of Hoth - Finish all levels with a GOLD medal
Death Star Trench Run - Finish all levels with a SILVER medal
Beggar's Canyon - Finish all levels with a BRONZE medal

GameShark Codes

Infinite Lives 80130b100008
Infinite Secondary Weapons 8010ca320008
99 Kills 80130b850063
99 Saves 80130b860063
All Levels/Ships 80130b4d007e
Extra Shields 81137e7c4316