I came across Class Dojo last week (don't ask me where, I'm lucky I can remember where I live at this point). Let me just say that I am SO excited for the school year just to finally have a system to track behavior - both good and bad. Classdojo.com is a free service which lets you acknowledge positive behavior in real time. A demo class is provided for you to explore the features right away.
Each student appears in the class with a little monster avatar
You can also give students access to their individual accounts so they can track their points at home. I'll be testing that out before doing so. Also, a rewards section looks like is in the works.