The 'Vop' (Vex OPerator) context in SideFX Houdini is the best 'node-based' graphical programming environment that I know of - however you're always working at

The 'Vop' (Vex OPerator) context in SideFX Houdini is the best 'node-based' graphical programming environment that I know of - however you're always working at something like a 'shader'-level. It's strength is in automatic use of SIMD and built-in parallelism (would not be a good choice to write an app in though!)