railspdf is a plugin that adds templating support for the pdf-writer gem to Rails. The project was last updated in January of 2007, but worked fine up until the 2.1.0 release of Rails which broke several things.
I’ve forked the project on GitHub and tweaked it to run under 2.1.0. I also added support for custom helpers. Your PDF templates can now call methods in helpers with the same name as your controller (which is the expected behavior anyway). I’ve also started maintaining a CHANGELOG.