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