Have a look at a goodie called Physics Painter which lets users paint physics objects onto any other object, and simulate gravity.
Physics Painter is a Houdini Digital Asset (HDA) that allows users to paint physics objects onto any other object, and simulate gravity. The result of the previous simulation can be fed into a new iteration of a simulation as often as required. The user gets control over a couple of parameters, which allows the user to get their desired result.
The tool can be downloaded as a part of Houdini Game Development Toolset here.