For a while now I've been working on a community puzzle site where you can solve a range of puzzles online using players that properly support pencilmarks and are easy to use, and where you can automatically compare and discuss puzzles with other users.
www.puzzlemix.com is the result of this, and it currently features regular new sudoku, sudoku x, kakuro and futoshiki (inequality) puzzles of a range of difficulties.
I've had over 50 puzzle books and magazines published, so the puzzles are of good quality. I'm also planning to add more sudoku variants throughout the year (although I already post esoteric variants on my sudoku blog at www.killersudoku.org.uk).
Anyway, if you take a look and have any comments then please do let me know (or post them here) so I can make the site as good as possible. It already has support for saving games at any point and a range of solving assistants to use, including for example an option to dynamically update clues whilst solving kakuro.
For more details, just check out the site: www.puzzlemix.com
Gareth