Cheetah3D provides the most useful features you need to get the job done, and organises them within a beautifully elegant user interface that's powerful, intuitive and quick to work with. Performing truly advanced operations in Cheetah3D will seem like second nature very rapidly. Cheetah3D packs a terrific punch for it's tiny price. In fact, the program provides a level of productivity that you will be hard-pressed to equal in software ten times the cost!
Current Version: http://www.cheetah3d.com/download.php
Unity Documentation: http://unity3d.com/Documentation/Manual/HOWTO-ImportObjectCheetah3D.html
Unity Scale Calibration
The current version of Cheetah3D does not support animating models using bones. One can instead export the models into an external animation application such as Dim3 Inspire and skin and animate them there before exporting into Unity.
Skeletal animation is in the works, per: http://cheetah3d.de/forum/showthread.php?t=850