Idiot's Delight is your source for free online solitaire. This site features applets for one hundred solitaire variations that can be played on all platforms (including Macintosh and Linux) that have a browser supporting Java applets. The games allow unlimited undo and redo and have several levels of difficulty to provide a challenging (but winnable) game for all levels of players.
Games include Freecell, Klondike, Spider, Golf, Yukon, and many more.
