Mediakiller is a project mainly written in Ruby, it's free.
A converter for MediaWiki markup to other formats
This is a very simple converter from MediaWiki to other formats (only markdown is supported currently).
Dependencies: mediacloth (gem)
Usage:
As an API:
MediaKiller.new(:markdown).convert(some_mediawiki_text)
At the command line:
mediakiller markdown some_mediawiki_file