Sudoku swept the UK in 2005 like some kind of infectious puzzle craze. The puzzles, also known as Su Doku or Sudoko, are interesting from a Computer Science point of view. Here's a program that solves them.
The rules are simple:
To use the solver, enter the values you do know, and leave the ones you don't know blank. You can use the tab key to move from cell to cell, and if you like you can enter something for the blank ones. A 0, a space, a dot - as long as it's not a number from 1 to 9 it's fine.
If you're looking for sudoku puzzles, you could do worse than visit The Guardian's page.