Maruku is a Markdown interpreter written in Ruby.

The other Ruby implementation of Markdown is Bluecloth.  Maruku is
much different in philosophy from Bluecloth; the biggest difference is
that parsing is separated from rendering.  In Maruku, an in-memory
representation of the Markdown document is created.  Instead,
Bluecloth mantains the document in memory as a String at all times,
and does a series of gsub to transform to HTML.

Maruku is usually faster than Bluecloth.  Bluecloth is faster for very
small documents.  Bluecloth sometimes chokes on very big documents (it
is reported that the blame should be on Ruby's regexp implementation).

Homepage:
http://maruku.rubyforge.org/
