This thread has gotten really long so I wanted to drop in and leave this disclaimer Forum Owner Robert Clough left at the beginning of this discussion:
Just wanted to throw in a quick disclaimer...
I want to be sure everyone understands the existence of this thread does not mean we (forum staff, etc.) encourage or condone people signing up on forums just for links.
The value in forums is in contributing and building your reputation.
It's also good to remember forum links provide little in the way of link popularity, they will add to your link graph and they will pass a little link juice but only a little. Search engine reps have publicly said forum links are not heavily weighted so keep that in mind when you're spending a lot of time registering at and writing threads in the forums listed in this thread.