This is a basic fastener called a roll pin, slotted spring pin, or tension pin. These are designed to replace M3 screws that various projects require in places where a alloy steel screw is overkill/too heavy etc etc.
The pins are hollow cylinders with a slot or slit placed longitudinally that allows for compression during insertion and should equally distribute stress throughout the screw hole. Please take note: One end of the pin is beveled. This goes into the hole first. This allows for easier insertion than a blunt pin...
The pin has been designed to have an outer diameter (OD) of 3.4mm and inner diameter (ID) of 2.0mm. The pin has a thickness (measured between the outside and inside) of 0.75mm.
Holes with a diameter of 3.1-3.2mm will work very well. 3.0mm holes will be extremely tight and you run the risk of breaking the pin or ripping the hole out of the piece your fitting. The clearance for a M3 screw hole is 3.2mm for a close fit and 3.4mm for a medium fit according to the charts I referenced.
There are .stl files for single pins in standard sizes from 4mm-16mm.
Unlike its steel counterpart this PLA pin is non-reusable. If you need to repin/remove a pinned part you will need to use a punch (as you would a steel pin) which will likely destroy/mangle the pin... This really isn't a big deal since you can print a tons, its just the nature of the medium we are working with and the forces it can sustain. Also, removal without a proper 3mm punch is likely to be a tad frustrating ;)
I printed using: .25mm layer, 4 shells and 120mm/sec @ 230 degrees on a MakerBot Replicator 2 with no support or raft (on duck brand blue painters tape). You might be better off printing @ .2mm due to the dimension changes I made in version 2 but it still printed fine at .25mm.
Printing single pins seems to work fine without overheating of the small part (at least on the MakerBot) but it doesn't take very long to print a dozen.