One more site to add:
Samurai Sudoku X (DJApe) (weekly, each Tuesday)
DJApe is also the provider of the Washington Post Samurai Sudoku puzzles. The answers of the Washington Post puzzles are available on the site above, and I guess you might have already known that.
Also, I've changed the case for your original post Colin. Just a few clicks of work using word processing software. Some viewers do feel uncomfortable reading all-uppercase texts.