2000's Kao can be a pretty unforgiving game if you die one time too many, so for those who want to enjoy the game, but have lost patience for it, I've compiled a list of cheatcodes. I knew one, I found 3 more online, then I figured out the rest (except one) using the Dreamcast codes. Tried all of them in game, they work. Hope they'll make your playthrough less frustrating and more likely to unlock that secret snowboarding level c:
[UPDATE: Apparently the secret bonus level doesn't unlock by itself, you still need to use the "unlock next level" code after beating the final boss]

Sources:
4 PC codes (mostly correct)[stare.e-gry.net]
9 Dreamcast codes[www.gamespot.com]

To input any of the codes below, you must go to the Map menu and hold spacebar as you input the entire code. Upon inputting a correct code, you'll hear a jingle.

UNLOCK NEXT LEVEL:
[Hold Space] Up - Up - Left - Down - Enter

UNLOCK ALL LEVELS:
[Hold Space] Up - Down - Right - Left - Up - L.Shift

+1 FLAG:
[Hold Space] Up - Right - Down - Left - Zero (must be numpad zero)

+1 GLOVE:
[Hold Space] Down - Right - Left - Down - Zero (must be numpad zero)

+1 LIFE: (this is the only code I wasn't able to figure out, hopefully someone else will)
[Hold Space] Down - ??? - Left - Up - ???

INFINITE FLAGS: (the flag counter stops going down)
[Hold Space] Up - Right - Down - Right - Left - L.Ctrl

INFINITE GLOVES: (the glove counter stops going down)
[Hold Space] Down - Right - Up - Right - Left - L.Alt

INFINITE HEALTH: (obviously doesn't save Kao from drowning/falling to his death)
[Hold Space] Down - R.Shift - L.Alt - L.Ctrl - Up - Left - Right

INFINITE LIVES: (the life counter stops going down)
[Hold Space] Down - Left - L.Alt - Left - R.Shift - R.Shift