Question2Answer (Q2A) is a popular free open source Q&A platform for PHP/MySQL, used by over 14,000 sites in 40 languages.
As of version 1.6.3, all development is taking place through GitHub. The collaborative development process is being managed by Scott Vivian. (Note that official releases are still distributed via the Q2A website.)
Please read the contributing page for more information on how to get involved.
Thanks and enjoy!