Unlock players:
Choose Battle Mode and any kind of match. Enter a User ID at the User Setup menu. Now at the Character Select screen, hold L + R+ Z and quickly enter the following:

Arii
A, Y, B, X, Y.
 
Briggs (alternate costume)
A, B, X, Y, X.
 
Briggs (alternate costume)
A, B, Y, A, X.
 
Carla
A, Y, A, A, A.
 
Chukklez
Y, Y, B, A, X.
 
Cruz
X, B, A, A, X.

D-Mob
Y, B, X, A, X.
 
D-Mob (alternate costume)
Y, Y, B, Y, Y.
 
Dan G
A, X, A, X, Y.
 
Deebo
X, X, A, A, B. 
 
Deja
X, Y, X, X, A. 

DMA
X, A, X, B, Y. 

Drake
B, Y, X, A, A. 

Drake (alternate costume)
A, B, B, X, X.

Funkmaster FleA
X, B, X, X, Y.
 
Headache
B, B, B, Y, X.
 
House
B, A, B, X, A.
 
Iceberg
Y, B, X, Y, X. 
 
Ludacris
X, X, X, Y, B.
 
Manny (alternate costume)
X, Y, X, Y, X.
 
Masa
A, X, B, Y, Y.
 
Method Man
Y, X, A, B, X.

Moses
B, B, Y, Y, A.
 
N.O.R.E.
X, Y, B, A, X.
 
Nyne
Y, X, A, A, B.
 
Omar
X, X, Y, B, B.  

Opal
X, X, Y, Y, B.
 
Peewee
A, A, Y, B, Y.
 
Peewee (alternate costume)
A, B, B, Y, X.
 
Penny
A, A, A, B, X.

Pockets
B, Y, X, Y, A.

Proof (alternate costume)
A, Y, B, Y, X. 
 
Razor
B, Y, B, X, A.
 
Razor (alternate costume)
Y, X, A, B, B.

Redman
X, X, B, Y, A.

Ruffneck
A, Y, A, B, X.
 
Ruffneck (alternate costume)
Y, X, B, A, Y.
 
Scarface
X, Y, A, B, Y.

Sketch
B, B, X, Y, A.
 
Snowman
B, B, A, A, X.
 
Spider (alternate costume)
Y, B, A, Y, X.
 
Steel
A, B, X, X, B.
 
T'ai
X, X, Y, A, X.

Zaheer
B, B, Y, A, A.


   

Thanks to Revolution readers CapSan and Martin Brodeur