Shadertool is an open source node-based shader editor that allows to easily make basic operations and transformations (binary operations, kernel matrices, rotation…), create inputs (Images, perlin noise, random values…) and inspect the outputs. It also provides a powerfull formula node that allows you to make your own GLSL shaders, using position, size of the picture and time.

Github URL: https://github.com/XDargu/Shadertool