PTC, a world leader in high end computer design software applications, makes their Creo Elements/Direct Modeling Express 4.0 software system available for free download. Featuring real-time realistic 3D modeling, an easy to use and understand interface, simple install, and quick start project capability, the free version supports up to 60 parts. By 'parts', they mean that assemblies designed with the system can have up to 60 distinct components.
According to the version comparison chart on their website, the free version is fully featured including all the basic functionality of the professional level paid system, but doesn't support the features that enable designing sheet metal parts as flat patterns with folds or bending.
It will run on both 32 and 64 bit Windows versions, but there doesn't appear to be Mac or Linux versions. You do have to register for a free PTC account in order to register and use the software, which is totally understandable.
The price is right. Check it out.