Links

Sites in English

The list contains mostly reference sites and forums. Sites that provide sudokus are not listed (after all if you have a good sudoku program you can generate better ones yourself). Some of the forums keep lists of sudokus with special properties.

Sudopedia: Everything there is to know about sudoku (well almost everything...)
The New Sudoku Player's Forum: Very active forum populated by sudoku addicts. If you want sudokus you can play with the "Patterns Game Results" thread (found under "Interactive Games"). It holds more than enough sudokus for a lifetime.
Programmer's Forum: The Sudoku Programmer's Forum. Not so active as the Player's Forum but still a lot of contributors and a lot of really good stuff if you want to code yourself. If you want hard (but not too hard) sudokus look for "Hard sudokus, chains are not required. Rarely, ALS is" under "Puzzles".
Eureka: Definitely good stuff (but you need a strong sense of humor).
Sudoku Assistenten: Forum related to the program with the same name.
Glen Fowler's Sudoku: Very complete tool for generating and analysing sudokus (the man page alone covers 15 pages A4).
Gordon Royle's 17 clue Sudokus: No sudoku with less than 17 clues has ever been found that has a unique solution. Gordon Royle has put up a web site to collect all known unequivalent 17 clues sudokus (about 47.700 sudokus till now).