The following cheats can be entered in singleplayer via holding down G, and then O, and then D at the same time (G and O and D, in that exact order), then type in a cheat, then press enter:

All car weapons: CW
All levels unlocked: ALLLEVELS
All on-foot weapons: ALLAMMO
Changes car to running person: STONEAGE
Double damage with weapons: DMDAMAGE
Flip car geometry: CARFLIP
God mode: IMMORTAL
God mode off: MORTAL
Max car ammo: CA
No car geometry: CAROFF
Reverse car geometry: CARREVERSE
On-foot radar: RADAR
On-foot reticle: RETICLE
Secret credits level: TEAMBEYOND
Targetting for on-foot weapons: TARGETS
Third person view: CAMERA
Toggle car wheels: WHEELSOFF