Sudoku
I'm kind of addicted to Sudoku - it's a great diversion when I work on boring stuff or just want to kill time, especially because (other than many other games) it's solvable if one thinks logically.
On my laptop I play the free iSudoku for Mac OSX, which sadly isn't available anymore online. I've also seen Sudoku for Mac (shareware, $9), though, and if you run 10.4 'Tiger', there's also a Sudoku widget available (freeware). For Windows, I quite like the look of Simple Sudoku (freeware) and Sudoku Works (shareware, £6).
Pretty cool are the different styles of Sudoku downloadable from the BBC News report about the inaugural world Sudoku championships.
Wanna play, too? Here's an online game, powered by javascript, thanks to sudoku.yosmany.net:
And here's a new one every day, courtesy of dailysudoku.com!