We're actually using OSQA on the backend which, as you might guess, is open-source.

http://osqa.net

That's some mighty fine skinning work you guys did there!

I recently created http://ribbot.com for this exact purpose.

And it's open source: https://github.com/barmstrong/ribbot

It's great to see companies adopting this model for support (validates my theory) and it looks like Stripe has stepped it up a notch design/feature wise which I think can inspire ribbot. Ribbot.com (hosted version) also supports custom domains etc.

Love the minimalist interface. Perhaps a little more differentiation between comments (becomes a bit hard to read when the comments are nested). We'll definitely use this.

Totally, a simple answers site like this is seriously in need. I wouldn't even mind paying $9/month for something simple and elegant and easy to brand like this.

There are a couple of help desk tools which can compile a Q&A style knowledge base out of customer support emails. Here's one I'm familiar with: http://www.desk.com/product#sec6

What about AskBot ?

http://askbot.com/

It has problems in the design department. It appears extremely busy, because your eye is drawn to the top and right hand side of the page, which is completely devoid of any content.

It's not helped by all the social media widgets and RSS links and Large Box Of Male Faces. And flags.

They really need to dump all the irrelevant junk and focus on the content.

Checkout http://ribbot.com