MeshLab is an is an open source, portable, and extensible system for the processing and editing of unstructured 3D triangular meshes. And can be found on source forge.
The nice thing about this tool is that it allows you to import an 3d model in about any format. Once imported you can optimize the model by reducing the amount of polygons.
Take a look at this video to get an impression of how awesome this tool is :
Code Noctis 