For cross-browser testing and to emulate sites on other devices till now we relied on extensions (like The User-Agent Switcher for Chrome) and added command line switches to chrome to change user-agent string in Google Chrome, now you don’t need all this, since it’s built right into dev tools of Chrome now, current dev version has it.

built-in user agent switcher in Chrome

Built-in User-Agent Switcher for Chrome

All you need to do is, open developer tools (use Ctrl +Shift+I shortcut instead) from the wrench menu>Tools>Developer Tools

Click on ‘Emulation’ panel, select Emulate Mobile and select the Model which you want to Emulate and reload the page for proper user agent spoofing and Viewport rendering.

Visit Chrome’s Mobile Emulation page and watch this video for more information.

[Thanks to François Beaufort]