Here are some cheats for the 3Dfx version of Mechwarrior Mercenaries: hold Ctrl+Alt+Shift then type:

su - invulnerability on/off
is - unlimited ammo on/off
oo - heat tracking on/off
it - "nuke" current target
re - destroy current target
in - gives you jump jets
cr - unlimited jump jet juice
li - ends mission succefully
be - enables Free-Eye mode