I used to be active on a few different forums, but these days it's mainly this one (most of the others have closed now, while this one's continued to grow). Apart from that I hang around Cycling News and it would be remiss of me to mention this site, which gave me the idea of cycling the Great Ocean Road a few months back (something every cyclist should do at least once).
I'll probably think of some others later.
Mostly I spend my time here, but do
hit the following sites (in no specific order)
(I'll only provide links to sites not previously listed).
rec.bicycles but be advised
this one has some major flame wars, and its no holds
barred so it can be very rough.