Great site for games to stretch those brain muscles. No downloading necessary, just click on the games you want to play. Each game gets more difficult in level as you progress.
Just as a disclaimer, I want to say I got this link and the "Encyclopedia of Life" from a Kim Kommando newsletter I subscribe to. a great resource for those (like me) who are looking for ways to increase their pitiful knowledge of computer know-how. Enjoy.
http://www.gamesforthebrain.com/