I recently made a tool for myself that I think has succeeded where so many of my other side projects have failed: actually saving me time even after factoring in the time it took to build it. I gave it the boring name "GLSL Grapher," because it graphs the output of functions written in GLSL. Here's my explanation for why this is useful, taken from the project page:
Developing and debugging shader code is a uniquely difficult problem. Graphing a function in a single dimension is a useful tool in the debugging toolset, but isn't always the easiest thing to do. For one, typical graphing tools all require you to translate your shader code into another format. This allows you to graph the output of your actual code.
I'm hosting the tool at /glsl-grapher/. The source code is available on Github.