Is there a Stack Overflow clone which can be hosted privately?

Update for 2015:

I have 3 teams switched to, which is a fast-riser in the space.

Among other benefits, it is hosted, and I can monitor all my spaces at once, rather than having to switch between separate instances of the site.

Here's another one that I found - OSQA is a free, open source Q&A system where users earn points and badges for useful participation.
(I haven't tried it though)

See this list of Stack Overflow clones.

Also check out : it's based on the same codebase as osqa, but more modular and customizable. E.g :

Looks like is what you're looking for

I suggest Qwench, simple and lightweight or flarum as my site