I just accidentally hit cmd-` (the key to the left of the "1" in the top number line) and it did something.
It looks like it's doing a "next window". I wasn't able to find any menu option that shows that as its shortcut, nor could I really find a "next window" menu option. (hotkeys almost always trigger a menu entry) shift-cmd-` also works as previous-window, a bit like the next-tab hotkey.
Works in Safari and Mail, it appears to be system-wide.
