The site Thingiverse is a great source for 3D printable models – and open source designs in general. The site is run by MakerBot Industries, but the contents is in no way tied to the MakerBot 3D Printer. Instead, the nature of the site is an open source community for sharing plans and instructions for building various contraptions – ranging from printable toys to a printable 3D printer.
The open source nature of the site is encouraged through the derive mechanism. This encourages users to build upon the work of others. Add the ability to tell the creator of an item that you too have built it, and the site grows from a digital warehouse to a proper community of makers.
Providing the categories newest, most popular and featured, it is easy to find relevant models to browse. The visual nature of the site (almost all models have a photo, or a 3D rendering) makes it easy to locate interesting items.
Common file formats are STLs, OpenSCAD files, but everything from written build descriptions to Inventor drawings are accepted. If you wonder if you have the equipment to build an item, you can check the list of required tools. These range from CNC mills and 3D printers to scissors and pliers.
The site also keeps a blog worth following. It features various models and provides tips and tricks in relation to 3D printing and general making.