At the Map screen, enter the following:

All Driving Upgrades:
Left, Right, Left, Right, A

All Fighting Moves:
Up, Down, Up, Down, A

All Gunplay Skills:
Right, Left, Right, Left, A

Bigger Car:
Down, Down, Down, A

Show Nick's Coordinates:
A, X, B, Y

Impound Garage Cars:
Up, Right, Down, Left, Up, A.

Play as Snoop Dogg:
From the city map screen, press R, Dpad Up, Dpad Right, Dpad Left, Dpad Down, Z, Z, A, Y, X, Y. Now make sure you go to a location and save the game before you exit to the main menu or the cheat will not work.

Another way to do this is to collect the 30 bones scattered throughout the city. Once unlocked, you have an hour to solve as many random crimes as you can for points. Depending upon your score, you get a different title, such as "Lap Dogg" or "Top Dogg."

You can find the first bone in downtown across from City Hall (Spring St/Broadway). Drive straight from the steps at City Hall and when you get past the bushes you will find the first bone. Once you have found the first bone all of the others will appear on the city map. However, notice that the bones rotate and point at the next closest bone.

Enter the following as your license plate, then hold down L + R when you select OK to activate:

M1K3      Play as commando 
HAWG      Play as biker 
TFAN      Play as gangster 
B00B      Play as female punk 
JASS      Play as donkey 
TATS      Play as tatooed lady 
PHAM      Play as butcher 
MNKY      Play as male punk 
BRUZ      Play as boxer 
J1MM      Play as corpse 
FATT      Play as police officer 
P1MP      Play as pimp 
MRFU      Play as asian 
5WAT      Play as S.W.A.T. 
B00Z      Play as street bum 
PIMP      Play as pimp 
B1G1      Play as your boss 
ROSA      Play as your partner 
HARA      Play as asian worker 
FUZZ      Play as Officer Johnson 
HURT_M3   Play as your partner in underwear 

Thanks to Revolution readers Partyhat, Roobadoo, Jon@step3, and johnson12!