Sumobots
All you need to know about Mini-Sumo here...
Rules
We will follow these rules: http://www.robotics-society.org/mini-sumo.shtml
Double elimination tournament
Parts
Here is a good list if you want to build your own. I bet you can build a bot for $30, if you already have an Arduino! I plan on doing this.
http://groups.google.com/group/tuftsroboticsclub/web/anatomy-of-a-sumobot
Kits
Parallax $240 - Comes with 2 bots. Kit works well, but expensive http://www.parallax.com/Store/Education/KitsandBoards/tabid/182/ProductID/319/List/1/Default.aspx?SortField=ProductName,ProductName
Sumovore Mini Sumo $98.50 http://www.solarbotics.com/products/k_sv/
Mark III $92 http://www.junun.org/MarkIII/Store.jsp
TAB Sumobot http://www.amazon.com/SUMO-BOT-Remote-Controlled-Programmable-Sumo-Bot/dp/0071411933