I created a website a few months ago that people here might enjoy:

http://www.logicgamesonline.com/sudoku/

It randomly generates sudoku puzzles so you get a different one each time you load it. The puzzles are graded for difficulty, and any rated below “Near Impossible” are solvable without guessing. There are pencil marks and background colors to help you if you get stuck, and a timer so you can see how long it takes.

There is also a sudoku tutorial to help you get started at http://www.logicgamesonline.com/sudoku/tutorial

Enjoy.