Aside from Unity, which of the many desktop environments support switching among the currently open windows using Super+1/2/3/.../9?
This is especially handy if you often switch among the same few apps, e.g. a browser, a file manager and an editor, but also have other apps open, so Alt+Tabbing to the desired window would take more time.1
That keyboard shortcut saves me probably 5 seconds per minute on the average, so ~40 minutes a day, and it's supported by Windows 7 as well. Adding custom keyboard shortcuts for the app doesn't work, because pressing the shortcut will launch a new instance of the app.2
1. The DE will order the most frequently used windows first, but you have to first look and see how many times you have to press Alt+Tab; while with Super+1 or Super+2, you always know which shortcut goes to one app or the other.
2. Some apps have an option to allow only one instance, but this is the exception rather than the rule, and even in that case, there will be a noticeable flicker of launching the new instance and seeing it exiting immediately.