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.
Output will primarily be to custom tools but also hlsl/glsl for painter, designer , Houdini, Maya etc. This is less about building a material library than it is about building surface shaders without the need for graphics engineers. We can compile an MDL file including custom functions down to our fragment language already…
Current maya-usd version already supports MaterialX, you'll even see the mtlx shaders previewing in the viewport of maya. Additionally Arnold renderer also started supporting materialx from USD. So you can already start using USD in maya with materialX, and render using Arnold. But when it comes to editing/authoring…