The Hacked by Del.icio.us! is run on MediaWiki, a free and open-source software package developed with Wikipedia in mind. Since the first version came out in January 2002, it has been used on all of Wikimedia Foundation's projects, as well as the whole of Wikia and many other sites.
An advanced version of MediaWiki, Semantic MediaWiki, is also featured on Referata's sites (including this one).
The current installed version of MediaWiki is 1.25.5 (d75270f); that of Semantic MediaWiki is 126.96.36.199.