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.
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.
The site currently includes sets for Structural, Basic Linear Motion, Electronics & Motors, Linear Rails & Bearings and Skins (plates & mounting screws). Numerous videos demonstrate how to put the stuff together into a variety of examples. Other examples show how Contraptor parts can be used with Sketchup and Thingiverse to develop more complex devices.
If you're looking for a great way to use standard parts on your project, consider using Contraptor.
Via Contraptor (Hat tip to Thomas)