Fully working printable M8 hex nuts with standard dimensions. They are derived from the wonderful ISO Metric thread library by Trevor Moseley.
Do you want to build a Prusa with printable nuts and washers? Just download them and do it!
You can see these parts in action in this video:
http://www.youtube.com/watch?v=xHMjs-O9nq0
More information (In Spanish):
http://www.iearobotics.com/wiki/index.php?title=Tuerca_M8_imprimible
UPDATE: 2012/Sep/06
http://www.youtube.com/watch?v=EIm0ZwCNhyA
The printable nuts are not only functional and cool. They can also be taken off very easily from the heatbed using a Wrench!! :-D