This is a massive object to test and calibrate your 3d printer for better overhangs with relatively flat angles. It's designed to test angles from 20° to 70° with increments of 5°. There are angle marks at the top of the object.
Print in ABS or PLA but PLA is recommended as it has much less warping.
Update 2012-01-06: I was asked to modify the height of the object so here you have of two versions now:
overhang_test.stl -> 27 mm height
overhang_test_half.stl -> 13.5 mm height
Update 2012-02-06: I printed this thing (small version) on my new printer which is an Ultimaker. Right out of the box (Cura standard settings) it handles all overhangs very well except for last two (65° and 70°). It's possible to print it, but the overhangs get "rippled".
Update 2013-05-02: Now featured in MakerBot's "Support Material" video on YouTube.
Update 2015-08-12: Now featured in Simplify3D's "Customizing Support Structures" video on YouTube.