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This free online image converter lets you convert your images to the Scalable Vector Graphics (SVG) format.
SketchUp - 3D Design
This 3D modeling program can be used for dozens of different purposes, one of them being designing for 3D printing. A free version of the software is available on their website, or students may buy the Pro version at a reduced cost. Addtionally, faculty members are able to get a free 1-year license for educational purposes.
Make Magazine is a great resource for all things Maker, but their 3D printing section is especially helpful to see news about 3D printing, see what others are doing with their printers, and learn more about 3D printing overall.