Pupil (US$5.00) is an app specifically designed for Macs with Retina displays, specifically Apple's Retina-equipped MacBook Pro range (rMBP). Although Mac OS X gives users five screen resolution options, they are cumbersome to access and change, with users having to enter System Preferences any time they want to change the screen's resolution.
Most users of a rMBP will be happy with the native Retina display resolution of 2,880 x 1,800, but more demanding (or adventurous) users may wish to regularly change their Mac's screen resolution to take advantage of the greater screen real estate rMBPs have to offer.
Pupil is a simple utility app that gives you access to varying screen resolutions (including 1:1 native pixel resolution for Retina displays) directly from the Mac OS X menu bar. Two clicks will change your screen's resolution, instead of forcing you to dig into System Preferences. Pupil also lets you customize your favorite six resolutions and label them.
Some of the resolutions are extreme and impractical, like 3,840 x 2,400 or 720 x 450, and it's no wonder Apple doesn't give direct access to them -- though you may be an advanced user who might use them for testing purposes. Resolutions like 1,920 x 1,200 or 1,680 x 1,050 (which appear as Mac OS X's More Space and second from More Space options in System Preferences) can be really beneficial, so it's very convenient to have on-the-fly, two-click access to them through Pupil.
Pupil is available through the Pupil website. It's not available on the Mac App Store, but it's fully compatible with Mac OS X 10.8 Mountain Lion. An alternative to Pupil is SwitchRes 4, which you can check out here.