While I sometimes play the NY Times daily sudokus, I really like Shichi variant over at DKM Games.

Also, shout-out to the original version of Indigo Puzzles, before they lost part of their codebase. They had a fantastic puzzle-helper: not only did it tell you where to look if you got irredeemably stuck, but (if you wanted it to) it showed you why you were looking at that cell, how to solve it, and how to recognize and solve similar cases in the future. Which massively improved my game!

They also had a cool tool where you could enter your own Sudoku numbers and then play the game on their site and use their checker and solver tools if you got stuck. So not only could you use it to test your own homegrown sudokus, but it was a massive help solving stuck puzzles that were on paper or another site without hint/help tools. He also had hashi, kakuro and several other puzzles online to play with.

Regretfully, Alastair lost part of his code base and the arrival of children disrupted his attempts to recreate it (good for him!). He can still generate puzzles and solutions, though, so now he sells them in books and for syndication. I miss you Alastair!