With ShaderToy you can experiment screen-space based shader development online. From simple texture deformation to complex raytracers and distance field raymarchers, you have the complete power of the GPU to experiment with all sort of techniques thru GLSL shaders.
ShaderToy comes with a big amount of presets that you can use as starting point to customize and create your own shader based effects. Great for quick prototyping or for full development. As it is online you don't have a complete realtime CG framework handy, just connect online and starting typing code, as simple as that.
When you need to design a function for your projects or simply want to visually inspect the shape of a function GraphToy can help you. It's a very basic applicaton
that allows you to enter up to six different 1D scalar functions and pan and zoom into them. GraphToy comes with the most usual scalar functions present in
programming and shading languages (like smoothstep or Perlin's noise) so that copy-pasting to your project is as easy as possible.
Play with some math and design your own sound and instruments, and play songs with them in the piano keyboard (with the keys on your computer typing keyboard). It
comes with a few examples (it might take quite some time to load, be patient)