I've used blender, but I deleted it. It was a very nice program, but when I tried rendering, nothing worked. It needed a program called Python, you should probably include that link with it. I tried installing/reinstalling it with minimal success. It wasn't very reliable.
It is also a program more built for 3D animation. If you had a robot with moving parts and you wanted to see how it would work, this is the program for you. You can 'run' your robot in real time, but it would only do the extent you designed it to do.
Yes, Google Sketchup is a big one. After a few minutes you can have a scaled sketch of your robot and can do various things with the sketch. It's pretty neat. I made a 3D model of my robot I'm working on for Batelle with it. Then I can work on it from home, because I know the general sizes and such of it, and can place little parts on it that are similar to sensors, whatever. http://sketchup.google.com/
A final program you might want to include is solid works. Peticularly built for mechanical stuff, but costs money:http://en.wikipedia.org/wiki/Solidworks