Stackoverflow is one of the best, easy to use and fast question / answer forums on the web. The best thing about stackoverflow is that you can participate in the ongoing discussions on the website and start new discussions without any formal registrations.
Update - Of the options mentioned here only Question2Answer remains actively developed as of date.
However Stackoverflow and it's sister sites deal with only tech topics. In this article we will tell you about 3 (three) PHP based Stackoverflow clone softwares (one ZEND PHP Based) which you can use to start your own Stackoverflow like discussion forums.
Qwench is a PHP/mySQL based StackOverflow clone that allows users to post questions and answers, has a points system similar to StackOverflow and lets users manage their questions, accounts and points.
Question2Answer is the closest that you can get to a Stackoverflow clone in true sense. It almost has all the Stackoverflow like features such as a fast integrated search engine, points-based reputation management, categories and/or tags (with auto complete). It also supports single-sign on for all sites based on Question2Answer, and supports Facebook connect.
Other features include e-mail notifications and RSS support, voting, comments and follow-on questions. The code is calable for millions of posts and can handle a lot of traffic with ease. It installs fast (five minutes or less!) and can be easily themed.
PHPancake offers more than what Stackoverflow has to offer. It is a complete community content based site which includes the the wanted mix of categories, tree-forums, linear-forums, article/tutorials submissions. PHPancake is based on the Zend PHP framework.