I got an openbeam set on clearance from Makershed, took one look at all the M3 bits, and thought "Sure, I can build something with this once I've figured out exactly how I want to arrange and connect the pieces, but how do I prototype things?

So I made a module that implements a flat plate with  a protrusion that locks into an openbeam slot (thank you so much, openbeam folks, for putting a dimensioned drawing online so I could copy the numbers into openscad and tweak them). The file uses that module to make a triangular corner clip and a crossclip (for joining beams not at an end). You could also make angled connections and lots of other stuff, but I haven't gotten to that point yet.

The clips fit quit tightly, at least the first few times you use them. You have have to change the clearances for your printer. And the crossclips need a brim to print, at least on my machine (and you have to trim off the support piece carefully). I doubt that they would work for a permanent installation, but it wouldn't be that hard to add holes for a bolt and a captive nut.