There are only really a handful of forums I keep up with these days.
There's Terra Firma, which is a Farscape forum, been going to bloody yonks.
I make the occasional post of The Escapist's forums, which is a gaming website.
I'm a member of Ratchet-Galaxy, which is a Ratchet and Clank fansite.
And I'm also in the Replica Prop Forum which is a site for making replicas of TV and film props and costumes.

Oh, and this place, obviously