@aeroengineer: What types of shapes are you trying to design, and what is your end desire with the products?
I can think of all sorts of smallish things that it would have been great to build for hobby projects in the past.
For the moment, did you see my blog Are We Losing the Secrets of the Masters? In thsi blog I mentioned an amazing book "Five Hundred and Seven Mechanical Movements" -- I'm thinking of starting off by replicating some of the gears used to implement these movements to play with them.
I play in the higher end cad systems, but I have downloaded a while back Autodesk's 123D. Last I checked it was still free, and it had reasonable capabilities. There is one open source cad that I remember looking at a while back, but it was the worst cad experience I have ever had (CATIA has 100X better user interface, and that is saying a lot). There is also Sketch-up, but I do not have experience with that one. I think that there are some mesh based, open source modelers, that while they will not make a lot of friends in the MCAD world, they will do some very organic shapes. I hear that the DesignSpark group will be coming out with a 3D modeler here soon.
In the professional world, some of the mid end systems ($2-8k) would be Rhino, SolidWorks, and Solid Edge. There are a few others as well such as Keycreator, but all of these are rather expensive.
What types of shapes are you trying to design, and what is your end desire with the products? These will also help you to narrow down your selections.