MediaWiki is free and open-source wiki software. It was developed in 2002 for use on Wikipedia, but since its codebase is open source, many other wiki sites have been built atop it, including this site, ProleWiki.
Other notable wiki sites that are of particular interest to our project here at ProleWiki are Cuba's EcuRed, Leftypedia, and Wikispooks (the former of which sometimes has right-wing analyses, but it sometimes does a good job at exposing the bourgeoisie)