This is about 20% of the ones I want to mention.
Formerly called The Institute for the Study of Accelerating Change, these folks study the acceleration of change caused by the exponential (or faster) progress in technology. The sponsor regular salons in several cities and hold an annual conference called Accelerating Change. The 2005 Accelerating Change will be focused on the technologies of Artificial Intelligence and Augmentation of Intellect. I'll be there.
They've been around since 1947, I've been a member since 1976. I particularly like their special interest groups.
The association for advanced (especially Unix-like) operating systems.