"This page contains a collection of small computer programs which implement one-player puzzle games. All of them run natively on Unix (GTK), on Windows, and on Mac OS X; they can also be played on the web, as Java applets."
There are 32 puzzles on this web page! Here's a screen shot of some of the games (they wouldn't all fit on my screen). Click on the image to see it in a bigger version.
You can play on the web by clicking on a game, or you can download them to your computer.
"The thumbnail image for each game links to a Java applet so you can play it in your browser. Below each image is a link to the Windows binary for the game, and a link to the manual. For all other platforms, the games are provided in a single bundle, so scroll down to the Download section to get them all." (link)
Don't be put off because these games look simple. They seem simple when you start, but will challenge you to master them all!
Thanks to Richard Byrne for the link.