I’ve been playing with Google SketchUp for a while as a free tool for doing electronics enclosure design (ie. MP3 Player cases). It seems quite capable although not really targeted at designs in the mm range. I think I will use a larger scale like 10:1 or 100:1 next time to see if that makes things easier.
Here is my first attempt at an AVR Butterfly model for use in case designs on the data logger and MP3 player. You can download in skp and stl format. To export SketchUp files as STL I have used the plugin from http://www.guitar-list.com/download-software/convert-sketchup-skp-files-dxf-or-stl