Clojure Dojo, every fourth Tuesday (of the month). We split into teams of 3 or 4 developers and all tackle the same problem, suggested by the attendees or the host (Toby) and agreed by the attendees. Each team has one computer and team members take turns at the keyword - a form of pair programming :)
After an hour / hour and a half working on the problem, each team presents their solution to the group.
It's a lot of fun and every skill level is welcome (and can benefit from the exercise)!