I've had a fully functional forum (http://www.chomisis.net/icarfora) for my RPG Icar (http://www.icar.co.uk) now for 3 months. During this time, the forum has proved invaluable to my design as I can add notes and ideas on there and immediately get feedback from regular players as well as irregular players.
If you have a game and a website to go with it, I would thoroughly recommend a forum. Make sure you keep posting to it, though, otherwise people will assume the game has died off!
A short note, but I hope it helps someone!
(Like the Forge, I use phpBB)