Two Disk Wobbler is a simple two piece toy that utilizes mathematics in its design to roll as well as it does. The first photo shows two of the 4mm thick version, the second shows a single 2mm thick version. I've uploaded "Disk 4mm.stl", the 4mm thick version, which works better on slick surfaces than the original 2mm thick version.
Two Disk Wobbler rolls good because its center of mass remains reasonably constant while rolling, thanks to the length of the slots in the disks used for attachment . The math involved in determining the length of the slots is not overly complicated, (there are a number of equations to solve for the center of mass, the equation I used involves triangles) and is simplified as:
Slot Length = Disk Radius * (1 - (1 / sqrt(2))), which is approximated by:
Slot Length = Disk Radius * (.2928932).