Not maintained any more. I cannot catch up with GitHub's layout changes

Demos: README.markdown, commit comments, issues, inbox reply.

Github Markdown Preview

WYSIWYM Markdown editor for github.com

Install github-markdown-preview.user.js

Tested on Firefox 3.5, Google Chrome 4, Opera 10, and Safari GreaseKit.

Google Chrome extension

Like UserJS, but with autoupdate and nice icon.

Credits

Based on GFM showdown.js