Open Space
An Open Space provides a unique and especially spontaneous way of coming together. With just one theme as starting point, it is mainly up to the participants what topics will be discussed and what the results will be.
Naturally, such an event will be professionally facilitated. At the start of the day, the purpose of an Open Space will be properly explained. The theme will be clarified as well. At the same time the location, ambiance and the facilities at hand contribute to a creative atmosphere. 
The fun part of an Open Space is that everyone can offer a suggestion for a certain topic discussion. This is followed by a voting of the most interesting topics. The attendees will then be divided in groups to discuss the selected topics.
The open character of an Open Space initially arouses the assumption that it automatically results in uncontrolled chaos. On the contrary. The day routine will be well directed, securing the various topics to all be discussed effectively. The present facilitators make sure important results will be recorded. Sessions going nowhere will be ended quickly or reframed.
Based on previous occasions, people seem to regard Open Space as a unique experience, a very enjoyable and especially productive day that should definitely be continued.
Wikipedia on Open Space Technology
Devoxx (formerly named JavaPolis) is an annual European Java
conference organized by the Belgian Java User Group (BeJUG). Theconference takes place on November 16th till 20th. With over 3200 attendees in 2008. Devoxx is the biggest vendor-independent Java conference in the world.