A browser plugin that lets you use Markdown in your comments and posts on Metafilter.com. More info, and installation, on the Markdown for MeFi site.
If you just want to use the plugin, see the link above. If you want to change the source code, follow these instructions.
You'll need to install npm, bower, and gulp first. Run npm install and bower install to install the dependencies. After that, run gulp default to build the extensions. (There's also gulp watch to rebuild automatically.)
Cross-platform source code is in the lib directory (this is most of the code), with a little bit of browser-specific code in the firefox and safari directories.
Most of the packages are installed through npm. We use bower for qunit, because the qunit that comes with npm doesn't include CSS files.