Plurk Solace - Free and Open-source version of stackexchange.com
opensource.plurk.com
opensource.plurk.com
;-)
Now, I'd guess them being released on the same day isn't a coincidence, but I think its interesting to point out they don't directly compete.
Our plan is to create a solution that we (our you) have 100% control over and that supports over 30 languages. Support is such a pain and Solace is mainly created to solve the issue surrounding support. Solace also offers an API that lets you integrate Solace deeply into your product.
Anyhow, the bottom line is that Solace is more targeted at startups and open-source projects - - while StackExchange is targeted at small business / enterprise market. That's at least how we look at it.