Integrating a forum.

Ok this is more me trying to gather as much information as I can before I start this. I have a good working knowledge of how integration works. However before I start working on a module to do this. I want to know if anyone has ever integrated the logins in pw database with a forum so users could login using their user names. If you have please help me out and drop any useful information you have on it. Once I get this done I will post a good guide on how to do it. Otherwise I guess ill be starting from scratch on it.