There's also LookDevX in beta, currently connected with Maya. Lucas Film said they're planning on releasing their MaterialX editor as open source. Houdini has MaterialX support.
MaterialX is able to store the materials in MDL. They work together. MaterialX is the glue that connects the geo to the shader. The shader itself can be in MDL, OSL or materialX. Its a guide to make it work. But there are multiple ways to use it. Similar to USD there are many flavors.