OS X: Apple stripped away much of the functionality of OS X's display preferences with the advent of Mountain Lion. Luckily, SwitchResX brings all of those features back, and adds a few of its own.
SwitchResX will install a new preference pane item for accessing all of the app's features, as well as customising what appears in the menubar drop-down. From the menubar you'll be able to change the resolution and rotation of all of your monitors, which can be handy when trying to sync up an extra screen, or for testing a web site's design on multiple resolutions. As an added bonus, the app enables running the Retina MacBook Pro at its native 2880x1800 resolution, which may not be very useful, but it sure is fun.
The app's best feature though may be its reintroduction of the "Detect Displays" option, which was a convenient one-click solution for setting up stubborn external monitors until Apple inexplicably removed it from Mountain Lion.
SwitchResX costs $US17 but has a 10 day trial, after which it becomes nagware.