What is it and what does it do? Is it worth getting? Thanks!
YafRay is a raytracer which has been made usable directly from Blender’s interface. It’s name is an acronym for “Yet Another Free RAYtracer” and it has some advanced features which are not found in Blender’s internal raytracer. It’s definatly worth getting as it can simulate the physics of real light rays and help you make more realistic renders.
You can get it here: http://www.yafray.org/
With the only exception of speed. With realism and those advanced features, you will need to get some patience as well. Google for free download.