Markdown is a wiki-like syntax that closely matches normal plain-text conventions. It's especially useful for creating README files, and other simple text files with minimal markup. GitHub automatically renders Markdown files as HTML.

QLMarkdown provides QuickLook support for Markdown files. So when you're in Finder, you can just hit the space bar when you have a Markdown file selected to see it rendered as rich text – assuming the file has a supported file extension.

QLMARKDOWN_VERSION='1.3'
curl -O http://cloud.github.com/downloads/toland/qlmarkdown/QLMarkdown-${QLMARKDOWN_VERSION}.zip
unzip QLMarkdown-${QLMARKDOWN_VERSION}.zip
sudo cp -a QLMarkdown/QLMarkdown.qlgenerator /Library/QuickLook
qlmanage -r
rm QLMarkdown-${QLMARKDOWN_VERSION}.zip
rm -rf QLMarkdown