Responsive UI for SharePoint 2013/2016 on-premises
You can download the package here: https://github.com/OfficeDev/PnP-Tools/tree/master/Solutions/SharePoint.UI.Responsive.
Once the files have been downloaded the solution can be installed using the Enable-SPResponsiveUI script:
Enable-SPResponsiveUI -TargetSiteURL https://portal.lab.local/responsive-site
Now I can resize the window and watch the various web parts wrap around each other: