Device Mode is an awesome tool for mobile web dev.
Google Chrome browser’s Dev Tools are fantastic. The “Device Mode” in particular is very helpful for testing mobile UX. It’s better than just modifying your UA (e.g. via “User Agent Switcher” or other plugins), because it has presets for lots of popular devices, and lets you set the resolution and even the device pixel ratio of the visible frame. Invaluable for testing mobile web UX, and testing RWD media queries.
This isn’t brand-new, but these things are easy to miss. Hope this helps someone out there! 🙂
P.S. And of course (though it bears repeating), a simulator isn’t a substitute for real device testing… and a desktop browser-based sim may not be as accurate as official vendor-provided simulators. But it’s convenient. YMMV! Good luck! 🙂