The Origins '05 "International Game Expo" is June 30-July 3, 2005. It's described at www.originsgames.com.
I went last year and had a great time. (Not sure if I'll make this year or not.) The place is huge (several large gaming rooms that seemed 100 yards long), with just about every type of game you can think of. (There's not a whole lot of electronic stuff; more board/card games, wargames, role-playing, etc.)
If you're used to software conferences, you'll find it a bargain. I've never seen a conference with so many simultaneous sessions (>100 at some hours).