The choice of modeling software really depends on the type of parts you want to make. But if you're planning to use a 3D printer, a solid modeling program has a lot of advantages, since you won't have to assemble your solids from surfaces, which can introduce errors. Blender is free, but it's pretty difficult to use, especially if you're focused on parts and not animation. Sketch-up is easier, but fairly limited in what it can do. You might take a look at the Cubify products from 3D Systems: Design (for mechanical models) and Sculpt (for more organic things); they're reasonably priced and fairly powerful.

Andrew Werby
www.computersculpture.com