Facebook Masquerade is a Chrome extension that allows you to rapidly switch between Facebook test users set up for a given Facebook Application.
Download/clone the repository. With Chrome extensions set to Developer Mode, choose "Load Unpacked Extension..." and select the repository. The plugin should now be installed.
Right click on the icon (which is now in your toolbar) and select "Options", or go to options through the extensions screen. Input your App Id and Access Token. You may input an App Name, though it's not used anywhere yet.
Click on the icon in your toolbar (currently looks like a Facebook Silhouette with a question mark over it). A list of test users should appear after a short delay; click on any name to switch to that user.
There is only limited error handling.