A GPLed Renderman-compliant rendering engine. Currently at version 0.7, with 1.0 ahead in distant (or not so distant) future. Originally written by Paul Gregory for Windows, it was later turned into an open source project (and now has many more contributors), and it now also works in many POSIX environments, such as Linux.
Aqsis is one of the best GPLed photorealistic renderers so far - maybe not as spectacular as some of the things that come out of BMRT, but those teapots look pretty nifty already¹ and some surface mapping things look pretty nice. It already implements most of absolutely required part of the Renderman standard, and some of the optional ones. However, reportedly it's not yet complete and still fairly slow.
Runs on Windows and Linux.
With BMRT's demise, this is probably the hottest "wild" Renderman renderer around...
Home page: http://www.aqsis.com/
¹ This is, of course, the primary goal.