Press @ or ~ to bring down the console. Now type in SISSY to enable

the cheat modes, then enter any of the following:

alamode                God mode
packnheat              All destructive weapons
payload                All ammo
iamsolame              All weapons, max ammo, invincible
Goto(level name)       Level skip 
piggytreats            More doughnuts
jewsforjesus           More cash
boyandhisdog           More dog treats 
jones                  More health pipes
iamtheone              More catnip
lotsapussy             More cats
swimwithfishes         All radar related items
fireinyourhole         Enables rocket cameras 
blockmyass             Body armor
smackdatass            Gimp suit
iamthelaw              Cop clothes
healthful              Full health and 4 medkits
whatchutalkinbout      NPC bystander pawns into Gary Coleman
oasma                  NPC bystander pawns into Fanatics
rockincats             Guns that use cats as silencers shoot cats
dokkincats             Disable cat shooting
ifeelfree              Toggle Ghost mode 
likeabirdy             Toggle Fly mode
walk                   Disable Ghost and Fly
slowmo                 Slow motion
ChangeDude()           Change to opposite morality
ResetCops()            Reset cops and wanted level

Thanks to Revolution readers Intermatrix(Figs) and Cairiel!