Sudoku Help provides puzzles for you, in the categories Easy, Medium, and Hard. It also enables you to enter a puzzle of your own. Pencilmarks are generated automatically, and you can show or hide them as you like. You can request a hint for the next step. For this app, both landscape and portrait orientations are supported. You can copy and paste sudoku puzzles as simple strings of 81 characters.

For those who are unfamiliar with Sudoku:
All digits 1, 2, ..., 9 must occur