GNOME Summit
The GNOME Summit is an annual hackfest for GNOME developers and contributors, traditionally held on Columbus Day long weekend. It is not primarily aimed at users or new contributors, but if you want to jump right into the deep end, it's a fantastic way to meet everyone and get involved. Unlike traditional conferences, the GNOME Summit is all about getting developers together and getting things done. While there are some non-hacking sessions, they are geared heavily towards many-to-many, interactive discussion and planning, rather than one-to-many presentations.
It has traditionally been held in either Boston or Montréal. See organization notes if you are organizing one.