The measure tool feels a bit forgotten and lacks a lot of features that other tools have. This is a proposal to make it a bit more useful.
Looking at the code it would probably need a lot of updating with a key map and so on, so maybe this could be concidered a hack, I don't know.
Press X, Y or Z to restrict the dimension to that axis (update: now also possible to toggle between orientations like in for example the move tool):
