I've lurked on a few other firearms related forums, but the only ones I have joined are THR, The Firing Line, and Rimfire Central. THR is my favorite and was the first one I joined because I like the respectful "High Road" atmosphere, and the members seem particularly friendly and helpful. I'm in no way new to shooting, but I'm still fairly green in my serious gun knowledge, and I've found THR a great place to learn without getting cussed out.
I also spend time on several 4WD (Pirate4x4 is my favorite), fast car, motorcycle, and forestry/arborist forums.