Sudoku Cup, a great site with daily sudoku league, sudoku variants (daily, manmade!), and online tournaments. Be carefull, some of the world's best sudoku players might be there - don't feel bad if your perfomance is not as astonishing as theirs.
World puzzle federation, the official body organizing Championships and online Grand prix.
A sudoku solver by Mr. Hewgill, that we've found very convenient.
Sudoku software for PC.
Boltds.net - great source of printable puzzles.