STEELER TALK
The following is a listing of some of the active Steelers forums that I've found. All of these have people using them on a regular basis. If you have a forum to add email me.

Offical Steelers Site - Probably the most active Steelers message board. Now divided into a bunch of different topics. Requires free membership.
NFL Fans - Mild usage. A lot of talk from other people with Steeler sites.
Chalktalk - A new one from the Trib's website.
Real Pittsburgh - A decent amount of traffic. Very knowlegable fans most of the time.
Black and Gold Brigade - From the official website of the most popular Steelers fan club.
Steelers Mailing List - A very active list. More information is available at the site.
Steelers ICQ List - Automically know when other Steelers fans are on the net via ICQ.