What is the button in the upper right of windows?
Solution 1:
Yes, it toggles the toolbar in all applications. In many applications, it's shortcut is Option+Command+T, but this is not true for all (Preview uses Command+B).
Solution 2:
For future reference, the Window Elements topic of the Apple Human Interface Guidelines is the definitive reference here.
Solution 3:
In Finder windows, the lozenge also shows and hides the sidebar.