K-3D is a free (GPL'ed) modeling and animation package, developed primarly on Linux which currently is being built around the Renderman API. Supports polygons and subdivision surfaces geometry, standard renderman shaders, unlimited undo/redo and a very nice and extensible architecture.
It currently has support for most(if not all) the existing renderman engine, 3delight, aqsis, bmrt, prman, renderdotc.
It gets updated frequently and has a CVS server to follow development closely.
See the official webpage for more screenshots and more info