Nymeros is an OpenGL Shader Language app that lets you test your GLSL programs directly on iOS devices.
You can view predefined Shaders in action, and change the screen resolution (to test performance) from the UISegmentedControl on the top. You can change the uniform color from the bottom RGB sliders, and you can change the time scale from the white slider!
You can also fork the shader program and customize it to your own needs!