Today we used Tinkercad to design 2D nets that fold up to regular 3D polyhedra, specifically the tetrahedron, octahedron, and icosahedron which all have triangular faces.
STL file: http://www.geekhaus.com/makerhome/day44_triangularnets.stl
Tinkercad link: https://tinkercad.com/things/66ZiQ7gjVMl-day-44-triangular-polyhedral-nets
Thingiverse link: http://www.thingiverse.com/thing:163459
Settings: Makerbot Replicator 2 on "Standard", which is .2mm resolution, in 11 minutes for the tetrahedron, 22 minutes for the octahedron, and 38 minutes for the icosahedron.
Technical notes: The nets are made to have .2mm (one layer) base thickness plus another .2mm (another layer) for the faces themselves, which are inset just a bit from the base layer so that fold lines develop naturally. Thicker nets did not fold very well and tended to snap along the edges.