Wings3D, Arbaro, POVTree. I use a lot of other freeware that isn't "open-source" per se, such as MapZone ( great texture making app) UVMapper basic, Ivy Generator, Deep Paint, etc. I would love to see a freeware app, open-source or otherwise that would generate animated "wave" or fluid, greyscale heightmaps in AVI or QT format that could be used in Carrara's displacement channel. Several commercial apps for water simulation use this method, but no freeware apps so far as I'm aware. 
   I have had pretty good success animating terrains in Bryce5 by importing a series of "wave" heightmaps prepared in PS and letting Bryce do the tweening and then rendering the result as an altitude mask movie from top camera view, This does generate a greyscale animation that is usable for displacement, but having an app devoted to this would help workflow on the occasions when animated water is needed. When I'm able to upgrade to C6 the new "Ocean Primitive" may make this a moot point. I haven't been able to try it yet, so I don't know.
                                                                                                                               Cheers, Dan.