Since it's launch last Monday I've heard a lot about WebPlatform. For those of you that don't know about it, WebPlatform describes itself as a "a new community-driven site that aims to become a comprehensive and authoritative source for web developer documentation."
So I was looking around and when I came to their forum (which they also title Q&A in various parts of the site), I was immediately struck by the SO-like interface. http://talk.webplatform.org/forums/
WebPlatform lists their stewards as: Adobe, Apple, Facebook, Google, HP, Microsoft, Mozilla, Nokia, Opera, and W3C, but I saw no mention of SO. In their FAQ, they say:
What technology is the site implemented on?
webplatform.org is completely built on open source technologies.
Web Platform Docs uses MediaWiki (which also powers Wikipedia) to run. It uses the Semantic MediaWiki and Semantic Forms extensions to apply more structure to the content and make it easier to edit.
Our Blog is built on WordPress.
Again, no mention of SO at all, however the resemblance is undeniable.