Question2Answer (Q2A) is a popular free open source Q&A platform for PHP/MySQL, used by over 16,000 sites in 40 languages.
Q2A is highly customisable with many awesome features:
Q2A also features an extensive plugin system:
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!