FreeCAD is a general purpose Open Source 3D CAD/MCAD/CAx/CAE/PLM modeler, aimed directly at mechanical engineering and product design but also fits in a wider range of uses around engineering, such as architecture or other engineering specialties. It is a feature-based parametric modeler with a modular software architecture which makes it easy to provide additional functionality without modifying the core system.
FreeCAD is based on OpenCasCade, a powerful geometry kernel, features an Open Inventor-compliant 3D scene representation model provided by the Coin 3D library, and a broad python API. The interface is built with Qt. FreeCAD runs exactly the same way on Windows, Mac OSX and Linux platforms.
It is still in early stage of development, so don't expect to be able to use it to produce work already. But, if you are curious about what FreeCAD looks like and what features are being developed, you are welcome to download it and give it a try. At the moment, much functionality is already present, but not much user interface has been created for it. This means that if you know a bit of python, you will already be able to produce and modify complex geometry relatively easily. If not, you will probably find that FreeCAD still has few to offer to you. But, be patient, this is expected to change soon.