FOSDEM is the biggest free and non-commercial event organized by and for the community. Its goal is to provide Free and Open Source developers a place to meet. No registration necessary.
Once a year, in Brussels, there is a gathering about free software.
This is not just a conference. A whole university gives their classrooms for a 3 days conference with tens of tracks, hundreds of speakers and thousands of attendees from all Europe.
After working in the development of GraphDatabases, and see how good are they to solve some problems, we created (and organized) the Graph Processing Devroom.
I also developed a small reference benchmark to compare existing solutions.