I have participated in many forums in the past. I miss them because of nostalgia but I also recognize that they probably wouldn't work as well these days.
Times have changed and traditional forums are clearly struggling to maintain the activity they once had.
A few forums I remember;
- Promotion of the forum (phpBB days)
- PhpBB communities
- FreeForums.org support
- Promotion source
- Promotion hotspot (I think that's right, I wasn't as active)