Game Developers Conference
I loved attending back then, and I love it now, even though it's changed a lot as the industry has grown.
My first experience at the GDC is archetypal of how I think a good conference works, with the interplay between presenters, audience members, information sharing, and networking, so I figure I'll tell the story here:
I was an unknown junior programmer at Microsoft at the time, and I got the company to send me to the Computer Game Developers Conference, the original name of the GDC. I went to a lecture by John Miles, where he was talking about 3D graphics. I asked some questions during the lecture, and then went up to talk to him afterwards because I was interested in the topic. We talked tech for a bit, hit it off, and he introduced me to Doug Church, Matt Toschlog, and others from Looking Glass, all of whom were thinking about the same issues. We all had dinner over fast-paced technical conversations about the details of how Ultima Underworld's renderer worked, graphics math, and whatnot, and lifelong friendships were born, and I was all of a sudden "in" the industry in a meaningful way.
It's that easy at a good conference in an industry where people are passionate about their disciplines. Find like-minded audience members and speakers, and go and talk to them about interesting stuff. You don't have to try to get up next to superstars like Miyamoto or Will Wright, you just need to find others passionate about games and your discipline, bring interesting and original thoughts of your own to share, and you will make connections and learn a lot in the process. This could mean going up to speakers after lectures and sharing your insights on the topic, listening to other attendees asking questions and talking to them, going to a roundtable and contributing yourself, or just meeting people in the hall or at a party (a party in which it's quiet enough to have interesting conversations, of course).
When you have a good enough idea or enough experience, submit a lecture or paper yourself and share your information, and when you're a speaker, remember to give the kid who comes up to talk to you after your presentation a chance, and the cycle continues and we all learn how to make better games.
todo: lots more work to do on this page