Redmine

The "Beast" http://beast.caboo.se/ forum built in rails
might be useful here. The goal is for it to be lightweight as
possible so may be easy to integrate.

Per project forums added in rev 529.

Permissions for forums management can be set in “Admin ->
Roles & Permissionsâ€Â.
Forums can be created on the project settings screen (“Forumsâ€Â
tab).
Once a project has a forum, a “Forums†link appears in the
project menu.
For now, posting messages in forums requires to be logged in.
Files can be attached to messages.