Enable code:  If you have a gameshark version 3.2 or 3.3, you can put this code on, just turn the code generator on.
EE000000 0000

Low Resolution multi player codes- you still need codes for each player, but the codes work for all levels!  So it's definetly worth it.

First Player codes:
All guns:
D013EE6F 001E
8013EE70 0007

Infinite Ammo (Right Gun)
D013EE6F 001E
8013DE5B 00FF

Infinite Ammo (Left Gun)
D013EE6F 001E
8013E5FF 00FF

Second Player Codes:
All guns:
D0140ADF 001E
80140AE0   0007

Infinite Ammo (Right Gun)
D0140ADF 001E
8013FACB 00FF

Infinite Ammo (Left Gun)
D0140ADF 001E
8014026F 00FF

Unlock level codes-  Use them in low resolution, then beat a challenge to save the game, then load your game in high resolution, and the new arena will still be there for you to use.

All levels unlocked (need gs 3.0+)-
50000E01 0000
800ACCEB 0001

Unlock individual levels:
Car Park:  800ACCEB 0001
Complex:  800ACCEC 0001
Warehouse:  800ACCED 0001
Ravine:  800ACCEE 0001
Temple:  800ACCEF 0001
G5 Building:  800ACCF0 0001
Grid:  800ACCF1 0001
Felicity:  800ACCF2 0001
Villa:  800ACCF3 0001
Sewers:  800ACCF4 0001
Ruins:  800ACCF5 0001
Base:  800ACCF6 0001
Fortress:  800ACCF8 0001

Have all gold medals at the firing range-  these can be entered with the expansion pack out, then save the game, run it with the expansion pack in,a nd you have all golds!  GrayVader made this awesome code!
810A22AC FFFF
810A22AE FFFF
810A22B0 FFFF
810A22B2 FFFF

High Resolution Codes

Must Be on Codes- to use any codes, you must use the code for your version GameShark

If you are lucky enough to have a Gameshark version 3.3, enter thisone line code:
FF1EAE00 0000

If you have a 3.0 or 3.2, enter this god-forsaken 50 line code:
 

F1000204 27BD  F1000206 FFE4  F1000208 AFA8  F100020A 0014  F100020C AFA9  F100020E 0018  F1000210 3C1A  F1000212 A05F  F1000214 375A  F1000216 AE00  F1000218 3C08  F100021A A07C  F100021C 3508  F100021E 5C00  F1000220 241B  F1000222 0040  F1000224 8D09  F1000226 0000  F1000228 237B  F100022A FFFF  F100022C AF49  F100022E 0000  F1000230 2108  F1000232 0004

F1000234 1F60  F1000236 FFFB  F1000238 235A  F100023A 0004  F100023C 8FA9  F100023E 0018  F1000240 8FA8  F1000242 0014  F1000244 27BD  F1000246 001C  F1000248 3C1A  F100024A A05F  F100024C 375A  F100024E AE00  F1000250 0340  F1000252 0008  F1000254 2400  81000204 3C1A  81000206 A05F  81000208 375A  8100020A AE00  8100020C 0340  8100020E 0008  81000210 2400

Once you have the enable code entered, you can use the following codes.  Keep in mind they work for all levels.

Partial Invincibility.  As long as you are looking at an enemy, they won't shoot you.  But they can still punch/hit you.
8025FE88 0001

No blood on enemies.
80246988 0001

Walk through most locked doors.  Fun to use on Agent, to explore the rest of the level.
8022E5FC 0001

If that doesn't let you walk thru the door you want, use this code:
8022E608 0001

Or this one, but DO NOT use them all at the same time.
8022E5F8 0001
 
Moon Jump codes!  When you are running off of something, like a wall, or running down stairs or a ramp, hold down the B button to fly in the air!  All of these codes are by Crocc.
 
 

P1:  D009C7E4 0040  8113D680 40F2  D009C7E4 0040  8013D633 0000  P2:  D009C7EC 0040  8113F2F0 40F2  D009C7EC 0040  8013F2A3 0000

FORTRESS ONLY  (The above codes do not work on the fortress level)  P1:  D009C7E4 0040  8113AE80 40F2  D009C7E4 0040  8013AE33 0000  P2:  D009C7EC 0040  8113CAF0 40F2  D009C7EC 0040  8013CAA3 0000