Cheats:

 CR3D    Credits
 CH3T    Dunno :)
 W4RP    Goes to highest level you've reached
 GHST    Invincibility

Codes:

 Code    Level      Code
 -------------------------------------------
 NTRO    1          Y0VR    17
 1STS    2          0V4L    18
 2NDS    3          T1N3    19
 TRSH    4          D4RK    20
 SW1M    5          H4RD    21
 W0LF    6          HRDR    22
 BR4T    7          L0ST    23
 K4RN    8          0B0Y    24
 B0MB    9          H0M3    25
 WZRD    10         SHCK    26
 BLKS    11         TNNL    27
 TLPT    12         H3LL    28
 GYSR    13         4RGH    29
 B3SV    14         B4DD    30
 R3T0    15         D4DY    31
 DRNK    16