I had fun with this; definitely a good mini game to learn more about XSS, although it's a pitty that you can cheat-pass a level simply by appending '/record' to the end of the URL. (Granted it's just a game)

I.e. https://xss-game.appspot.com/level1/record allows you to go straight onto level 2.

Anywhoo, HackThisSite is similar & worth checking out (albeit it covers a wider range of web app security issues)