Press '~' (without quote) during gameplay to bring the console and enter following codes to get corresponding results. You can toggle god mode and invisible cheat by typing the same code again. For give cheat ' # 'denote the item name.

Code - Effect

gametasksdone - Complete current mission
give # - Give indicated item
god - God mode
notarget - Invisible to all enemies
killall - Kill all enemies
killovums - Kill all ovums