Whyville

Whyville is a virtual world with over 100 games and constructive educational activities covering a wide range of subjects which helps children play and have fun while learning. 

It can also help teachers promote socially responsible behavior in an engaging and safe collaborative gaming environment. Membership is free.

Whyville works with such institutions as NASA, the Getty Museum, the School Nutrition Association, and the Woods Hole Oceanographic Institution to bring the educational content to its citizens.

http://www.whyville.net/smmk/nice