I love gamejams. You and your fellow participants all crowd into a room and build a whole load of games in a day or a weekend, all with a common theme. It's a lovely combination of community creativity and enforced delivery. Not only must you all come up with cool on-theme ideas for games… but you've got to get something finished before the time is up! It's a great way to flex your game-makin' muscles, and it consistently surprises me how much you can get done with a bit of focus. In short it is an inspiring experience. Make sure you try one!
I recently attended Gamecraft, an excellent 12 hour gamejam held in Dublin. About 80 people turned up on the day, with the theme announced at 9am leading to 12 hours of frenetic game-making by all involved, followed by 2 hours of judging & trying out other people's games. Yum!