Terrain2STL - http://jthatch.com/Terrain2STL/

Creates 3D-printable elevation maps of Earth's surface. I originally made it because I couldn't find a tool to make a particular 3D model I wanted, and then decided to open source it. However, to use the code you need to download ~80 GB of elevation data, which I thought would be a stumbling block for a lot of other hobbyists, so I put together a little web interface and hosted it myself.

It sometimes breaks even over a month from the occasional donation, and I think it's gotten me at least one job, but the best thing is the emails I get from users showing me what they've made with it. Seeing these really cool art and education projects from people more creative than me is pretty awesome!