Responsinator - Designed to test responsive websites on different devices.
SwitchResX - SwitchResX is a tool designed to manage resolutions of all your monitors and HDTVs.
Responsive Website Test - Test any website on desktop, mobile, and tablet
EasyRes - EasyRes is a fast screen resolution switcher for Mac OS X with live animated previews.
Multi Screen Test - Test your site at different screen resolutions & devices.
Resolutionator - Resolutionator makes it simple to use any of your display's available resolutions.