Here’s a keyboard shortcut that I use all day, every day (really), and if you aren’t aware of it, it could speed up your workflow considerably. You know how you can click on the Window menu in most applications to see what you have open and switch to a different window if you need to? Here’s what that looks like in Safari…
…and here’s Mail:
So if I'm working on "test message" there, and I want to go back to my Inbox, I use the Window menu, right? Or Exposé? Well, the shortcut Command-Tilde (~) switches between any windows you’ve got open in your current application, and I think it's much faster than either of those methods. You can use it to pop back and forth between messages you’re composing, different Pages documents you're working on, Finder windows, Safari windows, and so on. If you don't see what you're looking for, just continue holding down the Command key and tapping Tilde until you get to the window you want.
Not sure where the Tilde key is? In most cases, it’ll be on the left side of your keyboard just above Tab.
And for what it’s worth, Apple calls this the Command-Accent keyboard shortcut. I admit that I bear a little anger toward them for the fact that this shortcut won't work in full-screen mode, so I reserve the right to call it what I like. Nyah nyah.