The Game Developers Conference (GDC) is the official trade event "by developers for developers" of computer, console, mobile, arcade, online games, and location based entertainment.
Official Website: http://www.gdconf.com/
Added by tonys on July 3, 2007
For those interested in software security, consider attending local OWASP meetings. They are free...
http://www.owasp.org/index.php/Hartford
Ill be their for a few days provided im not snowed in at tahoe.
If your interested in games outside of hardcore - also consider Monday & Tuesday's Worlds in Motion Summit. It's part of GDC but focused a little wider on social games (facebook), casual MMOs, virtual worlds, and the "edge" of gaming.
retrogradeheart
I was lucky enough to go last year. Definitely check out the Indie Games Summit and the Experimental Gameplay Workshop if you're going this year.