Tin Pot
Guru
I think it's quite special - this CycleChat forum. I've not seen another forum quite like it. There's lots of activity and it's, generally, friendly.
I hover a bit in the Triathlon forum of the UK Runners World site. Not too bad. The Runners World US forum has a fair amount of activity and appears fairly interesting.
I've never seen a forum that is as well-presented, or has an equally good design/layout, as CC either.
I have a cunning plan - @Shaun should start a RunChat forum!
Tritalk good for advice but is a bit quiet, otherwise it's slowtwitch which has the odd insane thread.