WordPress.org Support Forums
The most prominent example of bbPress currently is the WordPress support forums which is rapidly approaching half a million posts from over 100,000 users, and it’s still as speedy as the day we started it. The advanced moderation and spam protection features of bbPress save the WP mod team tons of time.
Technorati Forums
A slick looking support forum for all things Technorati. Rocking design integration with the rest of the site.
Automattic Support Network
You can’t see the Automattic support network without paying, but they have essentially transformed bbPress into a password-protected enterprise support system with advanced privacy controls, all with a small handful of plugins.