http://sudoku.kikya.com/
It started as a project for my Web Programming class at uni, then I thought it was quite cool, and I added some features and graphics and put it online.
Basically you can register an account (nick/pass/email) and generate endless sudoku puzzles (3 difficulty levels, single solution symmetrical puzzles), and if you complete them your score will count towards the leaderboards.
You can also enter a custom puzzle, but that will not count as a score.
Other features involve user profile customization and sudoku printing. There's also a *chan-type text message board. Enjoy!
