This simple project has a Picaxe 28x2 and a speakjet daughterboard. I layed out the components of it in Google sketchup. Because LMR doesn't allow sketchup files as attachements, I'll refer you to my blog: http://ve9qrp.blogspot.com/2009/10/robot-plans-in-google-sketchup.html
The models of the caster, motor pack and wheels for those I had already bought were luckily already in the Google 3D Warehouse. I added the models of the various layers and, most importantly, of the PCBs that I had already made.
The servo is intended to rotate a Sharp IR range-finder, which is modeled as a black rectangle with some white dots. The white pillars represent bolts which run through the layers held in place by locked nuts on either side. This was largely to ensure that the battery packs would still allow clearance for the bolts and everything else. Some of the components are somewhat out of alignment, as you can see from the diagram, but even this level of accuracy was very helpful in ensuring that the final project would work.