(Unverified). Hit the TILDE key to drop the dialogue box and input the codes:

I CAN FLY ... Flight mode (no gravity player)
resetgames ... Ends current match (campaign)
hitmewithyourbestshot ... Adds extra enemies
takeittothelimit ... Ammo refilled to max
iwillsurvive ... Player invulnerability
closetoyou ... no-target mode on/off
letmego ... Stage Select (unknown effect)
sky ... Brightness increased (gamma positive?)
turbo ... Game speed increase

Submitted by hunter_joe_632, mail2sanju