Wolfe is a publishing toolchain. It converts Markdown text, HTML, XHTML, or XML to a formatted PDF, using (almost all of) CSS2.1 with extensions to support paged media.
It is heavily inspired by YesLogic’s Prince, which is an amazing piece of software, but not open-source or free.
Wolfe uses JRuby so that it can utilize two invaluable Java libraries:
Both of these are packaged as gems, available on the project page.