Under Lion (OS X 10.7) and earlier, a simple click in the menu bar let you change screen resolutions and, if you had multiple displays, toggle display mirroring. Mountain Lion includes an option, in the Displays pane of System Preferences, to enable a Mirroring menu (for AirPlay mirroring, not dual-display mirroring), but that menu is missing resolution options—and it appears only when an AirPlay-mirroring-capable Apple TV is available on the local network.
I've been accessing resolution settings by pressing Option and either of my keyboard's Brightness keys—a shortcut that opens the Displays pane of System Preferences. But a more convenient approach can be found in Milch im Gemüsefach's free Display Menu (Mac App Store link).
Like the old Displays menu extra, clicking Display Menu shows you a list of all possible screen resolutions, including HiDPI modes (and, for some displays, refresh rates); choose one to switch to it. If you've got multiple displays, you can also toggle mirroring, which means you can disable Mountain Lion's own Mirroring menu-bar option.
Oddly, despite the implication of the Display Menu app icon (shown at the top of this review), your screen-resolution options aren't listed in the main menu, but rather in a sub-menu. Given that the main purpose of Display Menu is to access available screen resolutions, I'd like to see those resolutions listed directly in the menu. Also, the initial release of Display Menu does not fully support Retina displays—you'll want to stick with QuickRes if you have a Retina Mac. Finally, it would be great if the developer would add a Detect Displays option like the one in Lion's Displays menu extra. But if you just want your screen-resolution menu back, Display Menu delivers.
Updated 10/31/2012, 9:00am, to correct details about Mountain Lion’s Mirroring menu.
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