RenderMonkey is described as '™ Toolsuite | AMD Developer Central'. There are more than 10 alternatives to RenderMonkey for a variety of platforms, including Online / Web-based, Self-Hosted solutions, Google Chrome, iPhone and iPad. The best alternative is Shadertoy, which is free. Other great apps like RenderMonkey are GSN Composer (Free), Shdr (Free, Open Source), The Book of Shaders (Free) and ShaderLab (Free).
The Book of Shaders provides a gentle, step-by-step guide through the abstract and complex universe of Fragment Shaders. Besides scores of insightful illustrations, the site includes a very intuitive live-coding editor for users to easily see changes as they code.
ShaderLab is the WebGL2 GLSL Editor for the browser. Code your shaders right in your browser with a live view to progress in the background. Then share and garner feedback through comments and check out other creators.
Shader Frog lets you create visual effects by composing two or more shaders together, and tweaking their parameters. You can build a complex effect out of simple components. You can then export your shader for use in your THREE.js game or application.