I'm launching a Q&A site similar to StackExchange.
Does anyone have any recommendations for free self-hosted Q&A software that is easy to customize?
I've seen this wordpress theme but it isn't free and, therefore, isn't what I'm looking for.
OSQA is also a nice implementation and is relatively easy to get up and running. What I don't like about it is how close it is to SE sites as far as design. Granted, its hard to come up with a better interface, but hopefully you make your's a little more unique :)
This is also GPL, so it's free as in freedom and free as in free beer.
Its written in Python/Django and should work even on the most basic shared web hosting.
Since you are using PHP/MySQL, I'd recommend Question2Answer. I've used it and I really like it. Although the default interface isn't as polished as Stack Overflow, it's still better than the other PHP Q&A apps I've seen.
Check out Stacked, an open source implementation of a Q&A site like Stack Overflow.
MSO has a huge list.
There are some pretty nice options available to you when it comes to Open Source, I've listed the best 3 that I'm aware of below for you:
You may also want to consider a SaaS solution if you're considering open source, as these solutions are automatically updated/updated/supported and require much less maintenance...but of course you have to pay for those perks:) Here is a list of powerful SaaS solutions that are also available for consideration.
Good luck with your Q&A community/platform:)