Diego Inácio has created several geometry tools for Houdini, using Python.
Nyx Sidekick - Preview
Here's a sneak preview of Sidekick, through which 3ds Max can communicate directly with Houdini.
Tea and Cookies in Houdini - Tutorial
Tea and Cookies in Houdini is a two part commercial tutorial in Houdini, going through all the steps of the scene's creation.
SideFX have announced Houdini Engine, an API that will facilitate Houdini integration into proprietary studio software.