Which script to install on Q&A site?

If you're planning to build a Question and Answers (Q&A) site, I'll recommend you question2answer (Q2A) PHP & MySQL script. Q2A: easy to setup, powerful to operate, flexible to maintain.
A Q&A site is the best way helps your online community to share from home to universe knowledge.
question2answer-script

Notes: All the attachments will be stored in database. So if your online community shares and saves larger storage, you'll need bigger database files ( requires higher performance hosting/server plans ).

Ready to setup:
1. Hit question2answer.org then download the latest version.
2.Create a database user/password.
3.Install yoursitename.domain/q2a/index.php
4. Join into Q2A community (question2answer.org/qa/) to get helps.

Easy to set up
Q2A takes 5 minutes (or less!) to install.
Easy styling with CSS themes.
Supports translation into any language.
Custom sidebar, widgets, pages and links.
SEO features such as XML Sitemap and neat URLs.
Fast and secure
PHP/MySQL scalable to millions of users and posts.
Safe from XSS, CSRF and SQL injection attacks.
Beat spam with captchas, moderation and/or flagging.
Core Q&A features
Fast integrated search engine.
Categories (up to 4 levels deep) and/or tagging.
Voting, comments, follow-on and closed questions.
Points-based reputation management.
RSS, email notifications and personal news feeds.

Reference source: question2answer.org