Free procedural Animation: K-3D

At http://www.k-3d.org you can download the Open Source K-3D application. While not as extensive as Houdini, it mimics the approach and workflow of the procedural animation toolkit for modeling, rendering and animation.

And the best part: available for Windows, Linux and OSX (when all additional problems are solved).

Everything is set up very flexible: you can use different rendering engines, such as Aqsis and YafRay, you can make networks of interconnected parameters, you get a flexible interface and you can extend it at will. Not bad.