Though Sudoku is fun even when played alone, it's actually a lot more enjoyable if you're competing with someone. IronSudoku.com is an online version of the game with a social flavor to it. The website basically allows people to chat with each other while playing. A new puzzle is posted at the website every day so you have 24 hours to solve it. I'm surely looking forward to solving some.
The interface of the game at IronSudoku is very clean and user-friendly and if you know the rules of the game, you can get started right away.
I would really like it if there was a way, let's say, for me to invite some friends of mine to IronSudoku for a competition online. I (organizer) should be able to pick the difficulty level of the puzzle for the event. At the start of the event, the system should start a timer ticking and pick a puzzle of the specified difficulty randomly. Every participant would receive the same Sudoku to solve from the system. And the Chatterbox should allow chatting only between the players in that competition. A visual indication should be given by the system as soon as somebody solves the puzzle. I'm sure this would be a lot of fun!
BTW as of now you have to be a Pro member to add other IronSudoku members as friends.