What I find most interesting, though, is that the USPS is now allowing custom-made stamps. Although these have a terms-of-service that looks pretty intimidating (Zazzle.com's TOS seems to be the least intimidating of the bunch, sadly.) That said, you can probably get away with a lot. I'd love to do a collection of stamps based on web-memes, myself. An "all your base" stamp, perhaps, or maybe domo-kuns chasing the kittens. Or there's always this...
So, what's your dream stamp? Feel free to use the design your own stamp tool, do a screen capture, and share the results with the rest of us in the comments!