Zignig has done a ton of hard work here orienting the various Mendel parts so they fit well on a build platform and putting it all into a single Blender file. However, as a windows user I had trouble getting the STL's out of the blended model. I had to edit the STL export script a little, but managed to export them all.
Here are all 41 pieces for a Mendel, a derivative of Zignig's work ( http://www.thingiverse.com/thing:1134 )
Update 2010-03-27: I've uploaded the parts individually as well, so you can see what each part looks like and download just the one you may need.