Shapeways

http://www.shapeways.com/tutorials/how-does-shapeways-work

1. objects must be closed.
2. objects must be manifold.
3. observe the maximum size and wall-thickness. (2mm)
   The maximum size of your object and the minimum wall-thickness depend on the production method that you are planning to use. Please see our full Material Overview for more information.
4. correct normals.

http://www.shapeways.com/tutorials/exporting-from-blender