A 3D modelling tool, distributed under GPL, works in X11/Linux and Windows. Currently in 0.1.x series, but seems to work at some level.
K-3D has, confusingly enough, nothing to do with KDE; in fact, it uses GTK+ toolkit. The K in the name doesn't mean anything.
K-3D is somewhat easy to learn; It has a very throughout tutorial, and the tutorial itself is not a boring HTML file with pretty pictures; actually, it's an interactive tutorial that is integrated to the program itself - the program guides you to use itself, moving mouse pointer for you and all.
The program uses multiple windows, with separate main/tools, document/undo, object properties and camera windows. The modelling is somewhat nifty (not as nifty as Blender, but definitely not painful, either!) Of the great interest is unlimited Undo and nice clipboard functionality.
K-3D is, as mentioned, a modelling tool; it doesn't actually render. It uses Renderman interface to do actual rendering - any Renderman-compliant rendering engine supposedly works; People say BMRT and Aqsis work like charm, Photorealistic Renderman apparently doesn't, but it's not really in the price range of us mortals anyway =)
(I will write of user experiences later on; I haven't yet even produced my first well-done image with it... I do have the impression that it's just a teensy bit unstable! Then again, finding open source Renderman modellers is a bit hard...)
Home page: http://k3d.sourceforge.net/