Type in the following codes during gameplay and then hit Enter:

Code       Result
gsaboff - Creates a technician (boffin) at the cursor. 
gsabog  - Creates a BOG weapon at the cursor that when you pick it up gives you extra defense and lets you lock doors. 
gsacmdr - Creates a commander at the cursor. 
gsaheal - Heals all colonists (not terminated) back to full health. 
gsakill - Kills selected characters (your own, or swaps, selects, and kills aliens). 
gsaknife- Creates a KNIFE weapon at the cursor that when you pick it up makes you invisible to aliens (they won`t go for you when they would normally see you). 
gsamedi - Creates a medical officer at the cursor. 

Push Enter and then type these cheats:

Code       Result
gsaroof - Removes all roofs (takes awhile) so you can see into rooms. 
gsastop - Stops both aliens and colonists from acting, causing them to just stand around; type it again to toggle it off. 
gsaswap - Swaps sides (allows you to control aliens); use following the kill cheat to eliminate selected aliens. 
gsawin  - Wins the level no matter what (so you can continue to the next). 

Thanks to Revolution reader Lord Edgtho!