It is frequently proposed (e.g, this from 2008: http://www.linuxjournal.com/content/wackypedia-wikipedia-for...) but I don't know of any that are even mildly successful. There are plenty of mirrors and forks, though. Naturally, Wikipedia has a list of them:

http://en.wikipedia.org/wiki/Wikipedia:Mirrors_and_forks/All

Not quite what you are looking for, but interesting resources nonetheless:

Deletionpedia:

http://deletionpedia.dbatley.com/w/index.php?title=Main_Page

See also the Semantic Web/Linked Data resource DBPedia (which tends to have poor uptime, so I am linking the DBPedialite here):

http://dbpedialite.org/