You may already know about it, but I thought I would mention it. There is a program called Pro/DESKTOP Express, from Parametric Technology Corporation, which also makes Pro/ENGINEER among others. Roughly 32 MB. Quite good, IMHO. You can design individual parts, then put them together into assemblies. It can do animations and even make it into a video.
You do have to answer a few questions to download it, but it's not too bad. http://www.ptc.com/community/free_downloads.htm
There are some tutorials with the built-in documentation, but more can be found athttp://www.prodesktop.net