EasyRes - EasyRes is a fast screen resolution switcher for Mac OS X with live animated previews.
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 Viewer - View multiple screen dimensions in one view
QuickRes - The best way to change resolutions on your Mac
Multi Screen Test - Test your site at different screen resolutions & devices.