Sudoku Assistenten is a free program that can teach both beginners and advanced sudokusolvers more about sudoku through its advanced hint and analytical features. It will let you both type in or copy/paste any sudoku into the program and then go on to solve it or accurately estimate its difficulty. It is also the only program that will let you create your own sudoku from scratch and then go on to share them on the programs online forum. Version 2.0 may include many bug fixes. A lot of new features copy/paste sudoku, more accurate rating, a unique analytical mode that completly changes the way you think about Sudoku, better interface, improved filetype.
Hard to believe this is free. Composing puzzles is both straightforward and highly addictive. Very user-friendly interface, and the ability to upload puzzles to the forum is an excellent addition. Best of all, it's free software that actually works - no disabled features or time limitations. Please download this software - you honestly won't find anything better.
Cons
Doesn't export puzzles as .txt files to use in other Sudoku software (as other programmes do) - but that's hardly a necessity. This software quite happily stands on its own two feet.