A (mostly) parametrically defined flying saucer. Hollow dome allows for including a pilot figure, eg. octopus, Colbert, etc.
UPDATE (8/1/11): Updated v3 with a hinged hatch on the top dome for even more excitement. Depending on your machine, clearances on the hinge might need to be adjusted.
UPDATE (12/16/11): Uploaded two versions of the hatch with tighter clearances, it should clip in more solidly. V4 is snugger than V3, and V5 is the snuggest, with bigger pins. I usually use a heat gun to adjust the hinge so that the hatch closes properly.

Print the top half and bottom half. Insert 3 small pieces of 3mm filament in the registration holes. Glue together.
If you're using the version with a hatch, print the hatch and snap it into place.