Jon Bentley suggested that good code can be enjoyed as good literature would be - with a brandy and cigar, by the fireplace. We agree, hence we meet monthly to discuss nominated illuminating works such as: algorithms research papers, Doom source code, articles from "Beautiful Code", and exploit techniques. Members are also welcome to discuss their our own projects.
The format is informal, but every meetup will have a volunteer presentation or a (non-compulsory-but-recommended) reading assignment and group discussion. Anyone is free to join (Not a programmer? Not a problem!), and post-meetup we usually head somewhere together for drinks and further discussions.