Patrick Min, a freelance scientific programmer has created number puzzles like Calcudoku and Killer Sudoku for many years. Recently, he decided to try and find the most challenging ones out there.
Our pals over at conceptispuzzles.com hosted the list for Patrick where you'll find both familiar puzzles and games such as Sudoku and Calcudoku as well as lesser known ones such as the Bongard Problem and Fill-a-Pix. Some of these puzzles can be solved right on their site, others can be downloaded or reached elsewhere. All of them, however, are promised to test your solving skills to the absolute limit and keep you busy for hours, if not days.