Is there a shortcut to switch between Add and Subtract brushes, in Sculpt mode? I'm working on some sculpting but I hate that I have to go to the Toolshelf and select Subtract, whenever I need that brush. So, while in Sculpt mode, how can I switch between Add and Subtract brush modes, with a shortcut?
You can press and hold Ctrl to toggle between sculpt modes. While pressed you will be in the opposite mode to what is selected in the toolshelf; i.e. if you have selected Add, then while pressing Ctrl you will be in Subtract Mode.

A list of all keyboard shortcuts available in Sculpt Mode.