3D printers are useful for a huge variety of things, but you need to programme them with a three-dimensional CAD drawing or digital design in order to get anything useful out of them. 3D printer software analyses 3D models, taking a series of cross-sections and working out the distribution of space and solid matter within each layer. The following resources offer free 3d printer files:see below.
MakerBot Industries (pioneers of the Thing-o-Matic) have formed Thingiverse, which is a community of people who create and share designs for objects that can be created using 3D printers, laser cutters and cnc machines, so that all can benefit from them.
The Sculpteo website provides a community platform giving users access to 3D designers, and allowing them to share models, upload 3D designs, and will undertake the actual 3D printing, shipping the object directly to your home or office.
Cubify allows users to download files for a small price. For each download a payment is sent to the creator/uploader of the file.
3D Systems, the company behind the Cube 3D printer have been involved with additive manufacturing and rapid prototyping since 1986 and they developed the STL file format which has become the industry standard for 3D printed object plans.
Most common CAD files can be converted to a format that will be recognised by 3D printers, including OBJ (Wavefront), PLY (Standford), OFF, SKP (Sketchup), KMZ (Google Earth), 3DS (3D Studio), AC3D, ASE (3D Studio), DAE (Collada), MD2/MD3 (Quake), Q3O (Quick3D), COB (TrueSpace), DXF (AutoCAD), LWO (TrghtWave), IGES, STEP (ISO 10303), VRML
Software packages such as Rhino, Maya, and SolidWorks are excellent for creating 3D files, and CAD software such as AutoCAD and Pro Engineer also produce good 3D printing results. There are a number of packages aimed at novices such as Google's SketchUp or Blender.
The following applications are free and will allow you to create your own 3D printer files.