wkhtmltopdf and wkhtmltoimage are open source (LGPLv3) command line tools to render HTML into PDF and various image formats using the Qt WebKit rendering engine. These run entirely "headless" and do not require a display or display service.

There is also a C library, if you're into that kind of thing.

How do I use it?
Download a precompiled binary or build from source
Create your HTML document that you want to turn into a PDF (or image)
Run your HTML document through the tool.
For example, if I really like the treatment Google has done to their logo today and want to capture it forever as a PDF:

wkhtmltopdf http://google.com google.pdf