If you have a printer of your own, there's nearly no limit to what you can make. You don't even need to be technically skilled in electronics or design.
People around the world are constantly uploading the digital files necessary to replicate their creations to a site that's especially popular among the 3D printing community: MakerBot's Thingiverse. While MakerBot sells its own line of 3D printers, there's no need for you to own one of their printers specifically in order to use these files. Any printer that can read the files will do.
Here are some of the most impressive, interesting, or non-obvious applications of print-it-yourself manufacturing you can find on Thingiverse.