I was pretty excited when I saw that someone had ostensibly ported Mario64 to an online flash game!
This game is really quite fun. At first I was a little disappointed that it's not actually the Nintendo 64 version, really just a flash-based Mario game with music borrowed from N64. However, it is still quite addictive!
Run around, jump, collect coins. Hit the spacebar to use your jetpack, but make sure you land to recharge before you run out of power!