Blender
Blender is the best free and open source 3D modelling program out there by a long shot!
The program is under constant development, have a huge community and have been used in many major films for video editing an D 3D Concepts. As said before Blender have new builds being uploaded hourly at graphicall.org and you can download all the official stable , Aplha and beta versions of the program at belnder.org.
The community is friendly and welcoming and you can fit right in at sites Suchs as blenderartist.org, Blendernation and BL Enderguru (great website for tutorials. I myself has been using the program for about six years now, and has used it for both personal and commercial projects: These can seen at (sixthlaw3d.wordpress.com).
To see the quality of the produced using Blender, watch elephants Dream, Big Buck Bunny and Sintel. All were created using blender, and other free software on Ubuntu.
The program has a brilliant set of tools, and the development team is constantly making improvements on the software to K Eep it cutting edge and competitive. This was a very, very short summary of this awesome program but check out it in more detail in the above links:)
Happy blendering!:)
Best 3D Modeling Software under Ubuntu