I babysit my neighbor's boys (ages 7 and 10) every other week after school, and I wanted to find a way to reward them for doing their homework. After looking up "chore charts" online, I came across http://www.handipoints.com/. The boys were able to create their own characters, and take them around "Handiland." It reminded me of a more kid-friendly version of WOW or The Sims.
The kids earn points for chores that I sign them up for. So every day that they finish their homework they earn a certain number of points by checking off their chore for the day. Those points allow them to purchase things in Handiland. The parent or guardian holds the master account, and is in charge of making sure no "point fraud" (my term) is occurring. :) They have a blast with it!
Parents magazine listed a few more neat sites for kids: Kidzui, Zookazoo, Franktown Rocks!, and Woogi World.