I was very impressed with the Substance Designer workflow, so decided to build my own node-based texture editor. After a month, here is the result.
It is only tested with Chrome, use it by visiting http://weesals.com/Releases/HTML/Surface/R3/.
Future goals are to better tag data types so they are converted correctly (ie. height-map implicitly converted to normal-map) and allow different types of data to be passed between nodes (points and meshes). And obviously add and fix nodes.