Foo Camp is the annual invitation only, no-structure, no plan, tent on the lawns, hacker event hosted by publisher Tim O'Reilly. Most recently: Foo Camp 06
For more info see: Wikipedia: Foo Camp
Opinions
Please note that BarCamp is not the anti-Foo. Foo is cool. Bar is cool. The important thing is that when good people get together, great things happen. RossMayfield (the janitor)
And as long as Foo remains cool and exclusive - it'll be great for those exclusive people to be all foo like with each other. Great, exclusive and foo-like. A new level of monetization by O'Reilly and cohorts. MarcCanter (the devil's advocate)