This is an add-on for Blender 2.79 and 2.80+.
Smooth Unlocked is a tool for Weight Paint mode, it lets you smooth the weights of selected vertices only for the unlocked vertex groups -- locked groups will not be affected.
This is different than the built-in weight Smooth tool, which doesn't take group locking in consideration -- it either smooths all of them, or just the active group. Using Smooth Unlocked lets you quickly define what groups you want to smooth by simply changing their locked status and then running the tool.
After installing the script the tool will be accessible in Weight Paint mode on the Weights > Smooth Unlocked menu.
For keymaps, the operator name is 'paint.smooth_unlocked'.
For quickly switching between vertex groups and locking/unlocking them, check out my other tool Popup Vertex Group List!
For questions or bug reports: