I loved the Hollow Demi-sphere http://www.thingiverse.com/thing:3068 but I was a little bit disappointed when I found out that it does not snap fit and falls apart by itself. So I remodelled it in OpenSCAD and then changed the angles of the cut out cones to get a snap-closing sphere that does not fall apart by itself. The only drawback compared to the Demi-sphere is that the two parts are not identical anymore, but they are still very good for printing as they are both flat on the bottom (the larger part is also flat on the top).

Because I'm on holydays and did not bring my RepRap with me I could not test the optimal angle for the best fit. So I prerendered some angles: 5, 7 and 10 degrees that I think would work well. Just make sure you print the two corresponding parts. metasphere_7degrees_larger_part.stl and metasphere_7degrees_smaller_part.stl for example.

FYI: for the illustrating screenshot I took an angle of 40 degrees to make the principle clear but I think it would have to bend way too much if you printed it like that so I prerendered only small angles that I think would work.

I'll be back home in a day and printing some of these will be one of the first things I'll do when I get back but I wanted to see if the thingiverse community is even faster with printing it :)

UPDATE: I printed the 7 degrees version. Results: The snapping is not snapping enough and the angles in which the two parts meet does not stabilize the parts so the sphere can be deformed too easily. I adapted the design by modifying the cones into straight cylinders deviating even more from the original shape but maintaining a flat base. I uploaded the new design and a stl files for a 15 degrees version that should snap nicely...I hope...well its only iteration 2 :)

UPDATE: these versions don't work too well...the second does not have a flat base...I followed another approach and came to a nice snapping ball: http://www.thingiverse.com/thing:3272