At the risk of blowing my own horn, if you're looking to play D&D (or any other pen and paper game system) Macray's Keep is somthing you want to at least take a look at.
Its much more than a PbP forum, with messaging, chat, character management (with a full d20 interface option), GM tools including campaign and scenario management and collaboration that is fully integrated with messaging and chat.
Membership is free (although there is a pay option) so at a minimum it is worth a look.
Macray's Keep