After an extensive search, I could find only three php based, open source, specialized, Q&A scripts for Apache web server using MySQL:
1. Qanda: http://www.qandasystem.com/
2. Qwench: http://github.com/anantgarg/Qwench
A comparison of the three scripts shows that:
a) Q2A has more features than the rest two
b) Q2A is most actively developed among the three- compare the release dates and the date of last commit!
c) Q2A has the best support system, if you need support, post it here, and you get an answer, mostly within a day.
As on date, Q2A is THE best in class! That is why I chose this script to my site.
I simply love Q2A and I am a fan of gidgreen!