I also like Shadron, it has quite a bit more capability (being able to use your own textures is pretty important! I can actually get work done with Shadron, can't do that reasonably with shadertoy). The downsides are the small community size and no automatic sharing / GitHub / OSS integration.

Other features include UI sliders, image saving, pretty decent library, unlimited flexible layers, way easier debugging, and lots more. I'm a huge fan, I'd love for more people to use it too.