This site gives information on how to set up your own Semantic Media Wiki.
Site explaining Semantic Media Wiki
Onto World is now renamed Semantic Web Dot Org.
Semantic Media Wiki is an extension of Media Wiki
Media Wiki is used by Wiki Pedia.
Sites using Semantic Media Wiki
Onto World (now renamed Semantic Web Dot Org April 2008)
Math Web wiki at www.mathweb.org 
Information newer than 2008: Wiki and Wiki Farm using Semantic Media Wiki wikiindex.org
Accurate usage is that Semantic Media Wiki is a plugin extension to Media Wiki and not a wiki engine itself.
See original on c2.com