If you liked to play with a Spirograph when you were a kid, you'll love to play with String Spin from zefrank.com (there are too many toys there to play with! Thanks, I think, to Grass Shack Events & Media for a great source of endless time-wasters!)
Here's another one, this time with color to play with.
Another geeky math game:
Conway's Game of Life (two thumbs up on this one, even though I don't really understand why it does what it does. It's just cool.)
And for us old fogies, a Roman numeral calculator.
Back to work now.