Here are a list of headless browsers that I know about:
- HtmlUnit - Java. Custom browser engine. JavaScript support/DOM emulated. Open source.
- Ghost - Python only. WebKit-based. Full JavaScript support. Open source.
- Twill - Python/command line. Custom browser engine. No JavaScript. Open source.
- PhantomJS - Command line/all platforms. WebKit-based. Full JavaScript support. Open source.
- Awesomium - C++/.Net/all platforms. Chromium-based. Full JavaScript support. Commercial/free.
- SimpleBrowser - .Net 4/C#. Custom browser engine. No JavaScript support. Open source.
- ZombieJS - Node.js. Custom browser engine. JavaScript support/emulated DOM. Open source.
- EnvJS - JavaScript via Java/Rhino. Custom browser engine. JavaScript support/emulated DOM. Open source.
- Watir-webdriver with headless gem - Ruby via WebDriver. Full JS Support via Browsers (Firefox/Chrome/Safari/IE).
- Spynner - Python only. PyQT and WebKit.
Headless browsers that have JavaScript support via an emulated DOM generally have issues with some sites that use more advanced/obscure browser features, or have functionality that has visual dependencies (e.g. via CSS positions and so forth), so whilst the pure JavaScript support in these browsers is generally complete, the actual supported browser functionality should be considered as partial only.
(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.)
Edit: Shameless plug: For those interested in a cloud-based solution, I've been building a cloud-based headless browser behind a REST API. Technically it's not headless, as it has full JavaScript/CSS/rendering support (it's based on Chromium), but for the developer, it's controlled purely by HTTP calls, so may as well be headless. Alpha testers are most welcome.