Most Everything!:
TRIGODS - Invincibility
TRISHLD - Shield restore
3DREALM - Power up all weapons
TRINEXT - Skip to next level
TRWARPx - Level Warp (x is the level to warp to)
TRIHOVR - Hover while firing
TRIBURN - Speeds afterburner up
MANIACS - 1000 Afterburner ammo
TRSCOPE - Oscilliscope
TRFRAME - FPS ticker
TRIFIR0 - Temporary invincibility
TRIFIR1 - PAC ammo
TRIFIR2 - ION ammo
TRIFIR3 - RTL ammo
TRIFIR4 - MAM ammo
TRIFIR5 - SAD ammo
TRIFIR6 - SWT ammo
TRIFIR7 - DAM ammo
TRIFIR8 - 200 Afterburner ammo
TRIFIR9 - Temporary invisibility

Tip:
After exiting a tunnel, return to the entrance and go through again to shoot down more enemy craft. They cannot aim at you, so there is not much risk. Also, on the last level's tunnels, use PAC's and keep shooting as you go through. This does not harm your ammunition unless you have collected another PAC from a weapons bunker which allows double shots for a limited number of shots.