Back when I had more free time (no wife, no kids, no business) I used to participate in a few forums. Now, I usually stay updated with the topics via Google Reader and the # of forums I keep up with have dropped to maybe a handful. They mostly come in handy for when I'm searching for specific tech related subjects and certain good threads show up high ranking in the search engine.
List of forums: VMWare's VMTM Forums, Ubuntu's Forums, and Webmaster Forum at Digital Point are the top ones that come to mind.