I recently launched a new Sudoku website that has a solver attached. You enter your puzzle and solve it online (or click the Solution button to get an immediate solution).
The site is free and offers unlimited puzzles of 5 levels of difficulty. It has a really smooth and efficient mouse interface for entering values and pencil marks. The same for keyboard entry.
Give it a try and let me know what you think.
Here's the url: http://www.SudokuByZol.com
Click the Solver button to enter your own puzzle.