This is a quick and dirty port of GitHub README styles to TextMate's preview window (used for styling Markdown previews).
It allows you to see READMEs as they will be styled in GitHub:
These files need to live in ~/Library/Application Support/TextMate/Themes/WebPreview/github To install quickly:
mkdir -p ~/Library/Application\ Support/TextMate/Themes/WebPreview
cd ~/Library/Application\ Support/TextMate/Themes/WebPreview
git clone git://github.com/kneath/github_textmate_preview.git github