A new release of the Wikitext gem, version 4.0.3, is out. This version adds support for Ruby 2.1.0 (and continues to work on Ruby 2.0.x; for older versions of Ruby, you'll need to use the 3.x series of the Wikitext gem).
See the release notes for full details.
About the gem
Wikitext is a fast wikitext-to-HTML translator with a syntax very close to that used by MediaWiki. It is a Ruby extension written with speed and robustness in mind using a Ragel-generated scanner and an extremely fast hand-coded parser, all in C. It has an enormous spec suite.
You can browse the source code repo here or here, or download the gem from the rubygems.org downloads page. For an overview of how to use the extension check out the docs here. If you have RubyGems then you can install it with either
gem install wikitext or
sudo gem install wikitext.