Really cool, didn’t know that. I just found this one I think it may have been mentioned before.
In edit mode when moving a vertex (G key) if you move it on the global X axis (Up/Down) then press the MMB it will be constrained to only move in that axis, or if you move it in X or Y axis then press MMB it will be constrained to move only in that axis.
You can always use the X, Y or Z key in grab mode, pressing once is global axis pressing twice is objects x, y or Z axsis. But I really like this MMB feature. Anything to speed up modeling is a plus!!!