If you're into Solitaire, then you can't really go wrong with Pretty Good Solitaire, which features an amazing 725 different types of solitaire card games.
These include classic games such as Klondike, FreeCell, and Spider, to original games that you probably won't find anywhere else (Eagle Wing, Empress of Italy, EastHaven anyone?). Although the unknown games do come with rules, all of the major variations are there including Aces and Kings, Patience, and Double FreeCell.
The interface is a bit ugly but fairly easy to use. Just scroll through the games and click on the one you'd like to try. All of them show the rules and high scores when you Ctrl-click or right-click on them. If you don't know which one to try, just click on 'Random' and Pretty Good Solitaire will throw a random game at you. The preferences are surprisingly detailed for a game of this type, allowing you to download different card sets, change the desktop background and add new players.
Pretty Good Solitaire contains enough games for every mood and if you can't find it here, you won't find it anywhere.