I helped the great folks at 9digits.de redesign their incredible Sudoku app - by far the best in the App Store. It also supports X-Sudoku, Killer Sudoku, and MathDoku - with hundreds of puzzles.

My favorite things about it are the mindbogglingly smart hint system, which actually helps you improve as a player, and the unique and very effective input system.

You can find more information at their website. Or, you know, you can get it right now (you should).