Contraptor is a DIY open source construction set for experimental personal fabrication, desktop manufacturing, prototyping and bootstrapping. Various Cartesian robots can be quickly assembled from Contraptor and used as a platform for projects such as XY plotter, mini CNC machine, 3D printer etc. Contraptor is an ongoing project under development. One of the original project goals is manufacturability at home with only basic tools, using inexpensive materials and parts readily available in hardware stores and online, with cost in the range of $400 and without the need for a custom-manufactured kit. In addition to DIY plans, we're also working on kits that can be manufactured in small quantities by anyone willing, from open source drawings.
You can create things with Contraptor without having the actual set. 3D models of Contraptor components are hosted on Google 3D warehouse and can be downloaded directly into Sketchup - a free 3D modeler that is gaining popularity because it is very intuitive and easy to use. If you prefer other 3D apps, DXF exports of "official" components are hosted on Thingiverse, a cool new site for sharing hardware designs.
Contraptor is mechanically compatible with 1" T-slot (such as 80/20 Fractional 10 profiles), pegboard, 1" grid beam (such as commercially available perforated steel tube), and likely other things with dimensions standardized in 1" grid.