MediaWiki is Wikipedia's own Wiki engine.
Not the most full-feature but certainly the most popular solution and one of the easiest to install and scale.
For information on mediawiki. go to http://www.mediawiki.org
for information on Documentation go to http://www.mediawiki.org/wiki/Manual:Code
Core development team is less than 10 people.It is based on PHP/MySQL.
GeoDNS, LVS, Squid, LighHTTPd, Lucene and Memcached can extend it.
MediaWiki code is also rather simple and well-documented, making it easy to adapt to specific needs.
Here is the link for MediaWiki Installation: http://www.mediawiki.org/wiki/Installation