ShoutWiki is a free wiki hosting service that uses MediaWiki as its hosting engine and is known for its various different, unique MediaWiki extensions. Many of ShoutWiki's staff members are also MediaWiki developers. See ShoutWiki's website or ShoutWiki's about page on ShoutWiki to find out more about ShoutWiki in general. ShoutWiki also has has its own Gerrit group which consists of ShoutWiki staff developers.
ShoutWiki uses many extensions used by Wikimedia sites, in addition to their custom extensions. The source code for most of these custom extensions can be found at Wikimedia's git repository (the official MediaWiki git). See Category:Extensions by ShoutWiki for a list of extensions written by ShoutWiki.