All work and no play has made Jørn the dull boy that he is. In fact, I think making a website is a much more fun way to use to a computer than playing games. But if you want games that are a little more than challenging than Solitaire, here are a few good sites:
Thinks.com - "the brain games, puzzles and pastimes website" (most games require Java-enabled browser)
Games, quizzes puzzles from Easton Language Education - a HUGE collection!!
Internet anagram server - Not a game, but fun if you like words (what English teacher doesn't?). Did you know that parliament is an anagram of partial men? Or, Clint Eastwood an anagram of Old West Action? Or Internet Anagram Server is an anagram of I, Rearrangement Servant?
Word Game of the Day at the Merriam Webster website
the MindBreakers - miscellaneous puzzles (fortunately with keys, as many of them involve math ...)