Simply the best Sudoku available.
For beginners, there is are two "easy" difficulty levels, as well as a useful hint feature that can be used at any time by clicking the "hint" button at the upper-right corner of any session. A single click highlights a blank space on the board that can be solved, and a second click reveals the answer in addition to providing a full logical explanation. This hint system is great for learning the techniques of the game, and helped me become a much better player.
The game allows the usual options found in most video game versions of Sudoku. You can opt to be informed of incorrect answers, as well as dupes. Veterans of the game will want to play the game on Master difficulty with most help options turned off, as this allows for the most "genuine" Sudoku experience.
The game does not require a powerful system to operate, and will probably run without issue on most systems. If you are interested in purchasing this excellent title, it is distributed inexpensively through Valve's STEAM software. A stand-alone version is also available.
Zen of Sudoku is the most full-featured, aesthetically pleasing, and user-friendly version of Sudoku currently available. If you are in the market for a premium edition of this wonderfully beneficial game, you can't go wrong with Zen of Sudoku.