Mikola Lysenko has created an app to turn an 8-Bit character sprite into a 3D printable model with his Shape Carving Editor using WebGL and the Shapeways API.  You can see some of his 8-Bit voxel sprite models in his Shapeways Store.

To use the Shape Carving Editor to make your own models take a look at Mikola's blog which has all of the instructions how to use the app along with all of the thinking behind the project.... 

The source code for the client is available on GitHub, so feel free to download it and try it out.