Viewport Resizer is a responsive design testing tool for Chrome that allows users to quickly test the responsiveness of any website. With just a couple of clicks, users can choose from various screen resolutions and emulate different devices to see how their website adapts. This extension is a helpful tool for web designers and developers during the testing phase, allowing them to easily share the defined environment of devices and breakpoints with their team and clients. However, it is important to note that this tool is not a replacement for testing on actual devices.