There are lots of online Sudoku versions but Sudoku Slam has a few cool features that you don't get in other versions of the game. And the coolest part is that these cool features are definitely not available when you solve a Sudoku the traditional way, on paper.
The first, most obvious, addition is that all cells in the grid show you the available options remaining and it can automatically update itself once you fill out a cell.
It also has things like "smart hints" that not only help you with your next move but also explain why you should do something so that you improve as a player, and the ability to highlight all candidates for a certain number.
Plus you can save your progress on a puzzle so that you'll be able to continue later.
Play the game