sudoku
I recently played a couple of sudoku games at a friends house and while I can’t say I’m addicted yet, I really enjoyed it and can understand the hype around it.
To play online from a web browser, I like http://www.websudoku.com.
I also found a sweet version of it for my cell phone at http://www.cellufun.com/Games/games.asp?game=Sudoku&from=websudoku.