Pro Action Replay Codes:
Show Developers Character Select Background - 00D495:01
Infinite Lives For Ren - 00C2F2:06 -Rid
Infinite Lives For Stimpy - 00C2F3:06 -Rid
Infinite Energy For Ren - 00C2F4:06 -Rid
Infinite Energy For Stimpy - 00C2F5:06 -Rid
Invincibility [Note 1] - 00D4CA:11 -Rid
Note 1: Get hurt once to activate, turn AR off to pick up items and when you reach the end of a level.

Game Genie Codes:
Invulnerability - 00E-E6C-B31
Start with 1 life - 013-4EE-E66 -Galoob
Start with 5 lives - 053-4EE-E66 -Galoob
Start with 9 lives - 093-4EE-E66 -Galoob
Start on a strange level - 001-D8F-5D4 -Galoob
Infinite lives - 001-B8E-3BE -Galoob
Infinite energy - 00E-ACB-3BE -Galoob
Stimpy starts with less energy after 1st life - 021-C0E-F72 -Galoob
Stimpy starts with half energy after 1st life - 031-C0E-F72 -Galoob
Invincible after first hit - 00E-E7C-E6E -Galoob
You still get hurt when you flash - 01E-ECB-803 -Galoob
Flash twice as long after getting hit - 99E-ECB-803 -Galoob
Flash longer after getting hit - FFE-ECB-803 -Galoob
Flash a lot longer after getting hit - 00E-ECB-803 -Galoob