MacOS – Keyboard shortcut to go to previous window

UPDATE: Spectacle is no longer maintaned. The website forwards users to Rectangle, see Jack's answer or https://github.com/rxhanson/Rectangle/releases.

I just tried Spectacle, great app and it's free.

This is what it can do with the selected group of windows.

You need to activate accessibility support.

System Preferences > Accessibility > Enable access for assisted devices

As per comments: The accessibility option in Mavericks is now in

System Preferences > Security and Privacy > Privacy > Accessibility

Just going into the main Mission control window will solve this, it will give a nice summary like view of all your open (fullscreen) windows. Another way to switch between fullscreen windows is selecting the "Window" menu item or the Safari icon in your dock and choosing the right window from there.

Edit

If you want to use a keyboard shortcut to accomplish this you can use an application that binds a keyboard shortcut to an Apple script, like FastScripts. The next step is placing the script in your /Library/Scripts folder and adding a shortcut to the script in the FastScripts preferences.

I'm not really good with Apple scripts, but this is a sort of working script that I made:

tell application "Safari"
activate
end tell

tell application "System Events"
    tell process "Safari"
        tell menu bar 1
            tell menu bar item "Venster"
                tell menu "Venster"
                    click menu item 14
                end tell
            end tell
        end tell
    end tell
end tell

The big problem with this script is this: The line click menu item 14 will make the first window that you see when you click on the "Window" menu item active, and changing it to click menu item 15 will make the second window active. However, obviously, this isn't included in my code, and I don't have the time to work on it today, so you can either ask someone else how to do this or wait until tomorrow.