I was going to add the MetaBrainz Community Discourse, but wasn't sure what category to add it under. Please advice.
But we/MetaBrainz (the foundation behind e.g., MusicBrainz) "launched" this Tuesday of last week:
It was originally using a custom OAuth2 plugin written by one of our Google Code-in students for authenticating against MusicBrainz.org accounts, but when we found out that it was still possible to change ones nickname/username when signing up on the site, we decided to make a quick move to use Discourse SSO instead.
It is replacing our old punBB-based forums, but we (that is: mostly I) decided to go for a "clean slate" approach rather than try importing old topics. With the traffic I've seen on the site so far, I'm satisfied that that was the right approach.
We're additionally using the tagging plugin and @Jake_Shadle's Jira Onebox plugin (though our Jira instance is still too outdated to actually work with this… It'll get updated. Soon™. ). We also have one of our community members working on making a Onebox plugin for https://musicbrainz.org/ entity pages, which we'll obviously be using, and they have additionally made a patch adding our Tagger Script language to upstream highlight.js (as well as getting that update pulled into Discourse), so we can start having users post their Tagger Scripts on our Discourse instance now too.
Far most users that have responded have really liked the change. There are a couple of people complaining that it looks too modern, but I reckon that you'll never be able to please everybody. We do still want to do some more customisation to make the site "look'n'feel" more "MetaBrainz-y", but that'll come soon enough. Baby steps.