This is something that I find pretty handy:
I am often ssh'd inter different servers or connected to routers, I usually have 4 Terminal windows open.
One of the big things is that by default they are not active / selected just by having my mouse over a specific window and you have to actually click on the Terminal window that you are actually using at that point in time.
So if you are dragging and dropping text between 2 Terminal windows you have to copy from source window then select destination window then paste.
Or you can enable the mouse over active stuff.
You can do this by opening a terminal window and typing defaults write com.apple.Terminal FocusFollowsMouse -string YES to enable or defaults write com.apple.Terminal FocusFollowsMouse -string NO to disable
This Hint could possibly be used for other applications too but I am not 100% sure about that.
In theory you could do the same with multiple Safari windows if you don't use tabbed browsing or if you wanted it for some reason.
Im not sure of the exact command but I would guess that you could use somehting like
defaults write com.apple.Safari FocusFollowsMouse -string YES