Devices with Retina displays are the future. Apple introduced iPhone and iPad with double resolution screens a long time ago, but the small sizes allowed software makers to ignore @2x graphics. Things changed rapidly when Apple introduced the new MacBook Pro with a Retina display. Now you simply can't ignore ultrahigh-resolution displays when developing new software.
To help developers and designers with older Macs, Apple has added support for additional display modes (named HiDPI) in OS X 10.8. You have to install Xcode and launch its Quartz Debug tool to enable the virtual modes system-wide. Then you will be able to switch between standard and @2x graphics using the Displays pane in System Preferences. This is the way you can test how a website or an app will look on the modern devices.
When you configure your display in System Preferences however, it takes plenty of time to switch from the standard to HiDPI mode and vice versa. It would be much better if the process was automated or simplified in some way. And here we go.
ResolutionTab is a minimal app living in the menu bar. It allows you to switch between real and virtual display modes right from the menu bar. ResolutionTab requires OS X 10.8 and Xcode to enable virtual HiDPI modes in OS X.