Pretty small, but in Rhino 7 (and presumably older versions), if you had clicked inside the Layers panel, hitting the Insert key on the keyboard would add a new layer. If you had a layer selected it would add the layer below it.
It was super handy because you often end up creating a bunch of layers at the same time. With the insert key, you could create and name all your layers without needing to take your hands off the keyboard.