Wikipedia must surely be on of the greatest innovations on the internet since the advent of Google and YouTube in my opinion.
The secret of it's success was to harness the power of user general knowledge via an extremely easy to use and read interface - a wiki. MediaWiki is a small package which contains everything you need to create your own wiki on any subject you choose. MediaWiki is already being used by several other projects of the non-profit Wikimedia Foundation including the MediaWiki developer site.
MediaWiki is basically a free server-based software which is licensed under the GNU General Public License (GPL). If you're intent on creating a big or popular wiki don't worry - it's designed to be run on a large server farm for a website that gets millions of hits per day. MediaWiki uses PHP to process and display data stored in its MySQL database. However, it requires no SQL database knowledge on your part and pages use MediaWiki's wikitext format, so that users without knowledge of XHTML or CSS can edit them easily.
MediaWiki contains everything you need to get up and running with your own no matter how big or small your project.
