For the past years, i have subscribed to dozens of Forums, but there's only two where i have been active on a regular basis:

The Criterion Forum: Forget IMDB, this is probably the most serious (perhaps too serious) of all movie boards, where film geeks debate about everything.
Music Mule: A board where underground/indie music is discussed, with direct ED2K sharing links. Britney spears fans look elsewhere.

Feel free to post yours!
Cheers