Let's say you're a little weird. Check that, let's say you are really, really, really weird. And also, you like random number generators, in the form of dice. You no doubt were first exposed this this concept as a child, when your board games used a six sided die -- a d6 -- to randomize your moves. Later, you were introduced to more complex games that used d12, d20, and percentile dice. And then you realized that there were... lots of dice.
You might, if you were a bit OCPD, decide you needed a full set. Okay, you're not likely to find a d11 (you can, actually), but you can find the first 10 numbers if you look hard enough. A true d3 or d5 is hard to find, but a d6 numbered 1-2-3-1-2-3 is easy enough; likewise a d10 can be modified into a d5. A d2 is just a coin. But a d1 is a bit tricky. Well, not really; you can just label a d6 1-1-1-1-1-1. Or you can be a real nerd, and buy/make/design one.
Most d1's are either a ring formed into a Möbius strip (link), a sphericon (link), or, if you're cheap, a sphere. Obviously, you need to write the number one somewhere on the device, to make it official. All of these are commercially available, and can be quite expensive. Amazingly, no one is currently selling a gomboc d1, but I'm sure they will once 3D printing technology advances; in the meantime, you can buy a weighted sphere that will roll so that the single pip always faces up (link).
And then you can start obsessing about the elusive d0.