As a teacher, I heavily rely on the ability to switch back and forth between mirrored and spanning mode for my Mac. I seriously hate messing with Displays preferences during class and fiddling with resolutions and such. Until this summer, Macs were pretty good at quickly switching between these modes, but with the arrival of Mac 10.8 Mountain Lion, the displays menu was replaced with the much more specific Airplay menu, which does not include the old Mirroring toggle. In fact, the key shortcut to enable/disable mirroring (⌘-1) forces a rescan of the displays, often resulting in a resolution change. Terrible!
Luckily, we can re-enable this feature easily with a free app from the Mac App Store called Display Menu by Thorsten Karrer. This app does exactly what you think it should, and it also enables the quick resolution switching that OS X had way back in the day. Everyone chained to a desktop with mirroring mode for their day job needs to get this app, posthaste. Check out more info on the Display Menu website.