The Vienna FreeBSD Security Hackathon is focused on security, but this is not a strict requirement. The basic idea is to get together with FreeBSD hackers to discuss and work directly on issues resolving as many as we can over the long weekend. Fixing bugs, writing code, adding features, improving the documentation, working on ports, and such are all welcome activities at the event. There is also a sightseeing tour planned on the Monday after.
Find out more here.