Level select:
While you are at the main menu press X, Y, B, Left, Up, Right, L, R.

Skip level with Silent Assasin rank and extra weapons:
As the level starts press R, L, Up, Down, X, A, click left stick, B, A, B, A.

Extra weapons:
Beat the following missions with a Silent Assassin rank to get new weapons.

Mission 1 - CZ 2000 Dual Pistols
Mission 2 - Micro Uzi Dual Submachine Guns
Mission 3 - Silverballer Silenced Dual Pistols
Mission 4 - Magnum 500 Dual Pistols
Mission 5 - Sawed-Off Shotgun Dual
Mission 6 - M4 Carbine Silenced Assault Rifle
Mission 7 - SG220 .S Dual Pistols
Mission 8 - MP5 Silenced Submachine Gun
Mission 9 - AK 74 Silenced Assault Rifle
Mission 10 - GK 17 Dual Pistols
Mission 11 - Micro Uzi Silenced Dual Submachine Guns
Mission 12 - PGM Silenced Sniper Rifle