Pixar has released their free, non-commercial version of RenderMan. It's available to download here. So far, only plugins available are for Maya & KATANA. Cinema 4D and Houdini plugins are being worked on, and there are a few other programs listed as potential plugin recipients (though it's highly likely they will get made). Of course, Poser exports RenderMan format since a billion years ago, so that's not really an issue, and those of us who use Rhinoceros3D also have the fantastic RhinoMan plugin.

I'm also putting the link in the free 3d software thread, but this, being a pretty big deal, I figured warranted its own thread as well.