A free tool to test responsible designs on target screen sizes side-by-side. Includes DevTools, click/scroll mirroring, and batch screenshot export.
There are a number of nice responsive testing tools like it that are available at the moment, including Sizzy, Polypane, Blisk, and Solis. Responsively has been working well for my needs when you want just a little more than DevTools Device Mode provides out of the box.
I'll sometimes try using hot-reload + side-by-side browsers (Chrome, Firefox, Safari, etc) or Browserstack for live testing during development, but I do like the simplicity of using these tools for responsive testing.