All of them — everywhere that isn't another control. Apple stole this design wholesale from GTK[1], but I think it's a superior paradigm anyway, so I don't really mind.
Is the list of messages on the left another control? It's clickable, but the whole thing is visually merged with the titlebar.
This has been part of Finder and iTunes since at least 2006, at a point when it wasn't the norm for anything in GNOME2 that I can recall.