4D rubiks Cube. There's a small (416k) download, but then it's probably the hardest puzzle in the world. It should keep you busy for a few hours.
Fiendish bouncing snake game. Requires Java and lots of spare time.
Monkey Lander. Flash based friday fun.
There goes the rest of the week. The 100 best online games. Ever.
Five Finger Fillet. Pick up that knife and play the game. Shockwave required.