Enter these codes while playing, without pressing pause:

Invincibility	R2, L2, Up, Down, X, R2, L2, R1, L1
All Weapons	R2, L2, Up, Down, X, Up, Square, X
Level Complete	R2, L2, Up, Down, X, L3, Circle, X, Circle, X
MIssion Select	R2, L2, Up, Down, Square, Triangle, Circle
Gravity	R2, L2, Up, Down, X, L2, L2
Lethal Charge	R2, L2, Up, Down, X, R1, R1
Punch Mode	R2, L2, Up, Down, X, Up, Up
Full Health	R2, L2, Up, Down, X, Up, Down
Bomb Mode	R2, L2, Up, Down, X, Up, L1
SloMo Mode	R2, L2, Up, Down, X, Up, L2
Megaforce	R2, L2, Up, Down, X, R2, R2
Nailgun Mode	R2, L2, Up, Down, X, L1, L1

Silenced Silverballers .45:
To get the Silenced Silverballers you have to get a Silent Assassin rank when you beat a mission.
(supplied by: dablinkerguy12)

Sawed-off shotgun:
Beat two missions with a Silent Assassin rank.

M-4:
Beat the St. Petersburg missions with a Silent Assassin rank.

M-4:
To get the M-4: in the "Motercade Interception" mission, use the fiber wire on one of the UN soldiers.
(supplied by: Nevinyrral1)