I participate in a couple of online music forums, including Last Plane to Jarkata and ILM (though only intermittently for the latter), but the forum I frequent the most is Readerville. Ostensibly about books--from spy novels to serious lit to cookbooks to you name it--there's also threads on movies, music, politics, television, and more. It's the most user-friendly forum I've ever seen, and it's also a real treat that all of the participants are de facto serious fans of writing, in the most general sense. That usually means when the conversations get going, they really get going. The cross-section of participants is also pretty varied, from college kids to grandmothers.
Readerville has been around for about eight years, and for a while it was the online counterpart to a print magazine, which folded a few years ago. Starting this week, the Readerville Journal has returned to an online form. Head over to the site to see the new features that have begun cropping up. Today, for instance, an interview with Ronald Harwood, screenwriter for The Diving Bell and the Butterfly.