Introduction to Picross. A square is either on or off and makes a graphic image. The only clue is a pattern above or to the right of the grid telling the length of the runs of on squares. This is now available on some game platforms and phones. Another Picross game, and another.
Somewhat related to Sudoku. The Washington Post has some of the best free Sudoku pages for a daily fix. BBC published an eight page Soduko mix of puzzles (pdf) available for download. Websoduko has a weekly similar feature. They also sell customized puzzle ebooks.
The top 50 Soduko sites.