An open source 3d scanner built around multiple RasPi single board computers and an equal number of digital cameras (between 3 and 5, inclusive). This is a fairly complex build, definitely not for beginners.
Open source greyprints for Braille polyhedral dice.
Github repo for the MIT RFID ring kit. Includes greyprints for fabbing your own rings.
.stl files for the Vampire, a quadcopter drone the superstructure of which is fully 3d printable.
Following the Terms of Service change at Thingiverse, archive.org downloaded the whole bloody thing and put it online. The tarball's 81 gigabytes in size, so I hope you've got some disk space...
The Universal Construction Kit is a set of open source components that you can fab on a 3D printer, which allows the user to hook together different construction kits without duct tape or twist-ties. The components make it possible to use Lego, Duplo, K'Nex, Lincoln Logs, and other construction kits in the same project simultaneously.
The whole set can be downloaded as a .zip file from the website.
The .stl files carry a Creative Commons By Attribution/Noncommercial/Share Alike v3.0 license.
Note that the .stl files are in inches, not mm. Double-check the settings of your 3D printer before fabbing!
A how-to for building a 3D printer which uses organic or biological materials for its feedstock instead of inorganic materials.
E-Nabling the Future is a community of makers, 3D printing hackers, and makers who design, build, and manufacture prosthetic hands and arms for children.
This site does pretty much what it says on the tin - it lets you design your own sex toys using a web browser. You can download the STL files and fab them on a 3D printer for personal use.
Highly extensible control software for 3D printers. Can run on a standard sbc like a raspi. Works with many printers.