Sudoku Fans@China site has introduced some really nice online solving features, which is rare to see at other sites.
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 :-
)