Shader Fiddle is an OS X application that allows for live experimentation with GLSL Fragment shaders. It is designed to be (mostly) compatible with shadertoy.
All of the uniforms are added automatically to the top of the fragment shader, leaving just the main method to be created.
There are several uniforms available for use:
uniform float iGlobalTime // Time in seconds
uniform vec2 iResolution // Viewport resolution
uniform vec4 iDate // {year, month, day, time in seconds}
uniform sampler2D iFFT // FFT data (256x1 texture, data is in the red channel)
The fragment colour is output with the variable
out vec4 fragColor
Shaders are #version 330, so GL_FragColor is not available for use.
The FFT information is taken from the current system audio input source.