I'm looking for something as similar as possible to ShaderToy, with one extra feature: the ability to load custom images to use as textures.
For those unfamiliar with ShaderToy, it's a web-based editor for GLSL fragment shaders. Your shaders are rendered on a 2D plane (not full 3D models), which is perfect for my purposes, as I want to use this for testing out shaders for 2D games. ShaderToy also has a nice bonus in that it allows you to share your shaders and see others' shaders.
ShaderToy itself has worked great for me in experimenting with shaders, but the lack of ability to load custom textures is limiting. I'd really like to try my hand at writing shaders that use normal maps, for example, but I can't reasonably do that without being able to load the normal maps as textures.
I don't have any strong preference for shader languages, but I'm guessing virtually all such web apps will use GLSL due to the existence of WebGL. Though I'd be more interested in using Cg shaders in the unlikely case that such a web app exists.