1) Multi Player ( http://www.sudokufans.org.cn/pk/ )
Several players can solve same sudoku and see how they rank against each other. (yes, I know some other sites have this for Classic Sudoku, but not "free").
Here you can choose any of 30+ variations as well.
2) Cooperation - Solve Sudoku Together ( http://www.sudokufans.org.cn/pk2/ )
In this game, two players (partners) solve one sudoku together.
The online solving interface is neat, most of the text are in English. So lots of fun.
Be forewarned : It is bit addictive.
Registration is required, but it is free and harmless
