Waste Time at Work!
Fancy Tickling, Hot Shit, Nerd | (0)
Crayon Physics
Probably the most fun game ever (and educational too!): Crayon Physics
Two caveats with this one, however: it has to be downloaded (something probably not available at work for most people) and requires the devil's computer (a PC). If either of these things are unavailable to you, please read on.
Magic Pen
If you're not allowed to download at work, don't fret: an almost-identical game, Magic Pen, is just as sick/awesome, and you can play it online.
You Have to Burn the Rope
Finally, there's the tightly-named You Have to Burn the Rope. This one is dope. If you don't believe me, this guy ...