Opens Salesforce1 Mobile in a new phone-sized window.
This extension adds a small phone icon in the omnibar if you are on a current salesforce.com instance. Clicking the phone icon will open a window with roughly the same resolution as an iPhone 5 to the one/one.app for that Salesforce instance. Handy for doing quick demos of Salesforce1 mobile applications on a desktop browser.
UPDATE: It appears in Summer '16 Lightning may have to be enabled to run Salesforce1 in a browser. I believe it only needs to be enabled on the org/instance level.
I'm looking into this, but if it is determined at the org level it is out of my control.