All the studnets at my school are addicted to this game. It's pretty simple, easy to understand, and really fun to play. There are other verisons of it, but here's the first one to try out.
http://www.ninjakiwi.com/Games/Tower-Defense/Bloons-Tower-Defense.html (http://www.ninjakiwi.com/Games/Tower-Defense/Bloons-Tower-Defense.html)