Here are a list of headless browsers that I know about:
- Watir-webdriver with headless gem - Ruby via WebDriver. Full JS Support via Browsers (Firefox/Chrome/Safari/IE).
- Spynner - Python only. PyQT and WebKit.
(note: original version of this post only mentioned HtmlUnit, hence the comments. If you know of other headless browser implementations and have edit rights, feel free to edit this post and add them.)