Parametric Make: Math Monday Linkage Bar
Recently Make: has been running a series on linkages:
http://blog.makezine.com/2012/07/30/math-monday-linkages-an-introduction/
for which they provide the MoMath Linkage Kit as a PDF that you can print out or laser-cut.
Or, if like me you have a 3D printer and no laser cutter, you can print them out :)
As a bonus, the bar lengths are engraved in the bars with triffid_hunter's 8-bit number function.
Note that so far, I have satisfactory bars but no connectors yet; suggestions welcome.