Beyond Cyberdrome is an event created by SMS at Evolution, the 1996 Eastercon, and held annually ever since. It traditionally involves small robots made from scrap toys, Lego, and K'nex, competing to beat the crap out of each other over several rounds to win prizes like the Victor Cyberneticus and the Most Brilliantly Engineered.
Each year has a different theme, for example Pirates (Arrr!) or 2007, and the audience and participants will often be asked to dress appropriately. The aim is towards maximum participation and fun rather than technical excellence. It is not a rip-off of Robot Wars and participants aren't expected to spend lots of time and money building and perfecting their robots.
Since 1998, the Beyond Cyberdrome organisers have also run a workshop throughout the con loosely inspired by Chaos Costuming, where people can build robots under the guidance of experts to take part in the Beyond Cyberdrome event. The workshop has been variously known as the robotics workshop, BC workshop, chaos robotics, and the Susan Calvin Robotic Assembly Playroom (SCRAP for short).
Another spin-off from Beyond Cyberdrome are the Sprokettes, a team of gorgeous women (and the odd ugly bloke who manages to bribe his way past the selection committee), who act as Beyond Cyberdrome cheerleaders, or stage assistants, or something like that.
The official Beyond Cyberdrome web site with details of past events and how you can take part next year is at http://www.beyondcyberdrome.org.uk/