The general idea is to invite 4-ish people onto a panel and have them talk on some subject. The invitees are not told more than a day in advance what they'll be talking about, and it's considered against the goals of the conference to do any real preparation on the subject. The panels are cross-disciplinary, so the it may be Roger Ebert on a nanotechnology discussion or Henry Kissinger on a talk about contemporary Jazz (though it's equally likely that they'd be talking about silent movies and international diplomacy, respectively). Anybody been? I have not yet, but keep meaning to.
Conference on World Affairs. Founded in 1949, free and open to the public, discussing many many issues that affect us today. Notables who've spoken at it include R. Buckminster Fuller, Henry Kissinger, Roger Ebert, Andy Ihnatko, and Arthur Miller.