Project Fi users have been able to enjoy access to Google's WiFi Assistant feature since Google launched its MVNO, but soon all Nexus owners will be able to get in on the fun. That means you can connect automatically to open WiFi hotspots via Google's secure VPN. There are some geographical restrictions, of course.
You'll need to be using a Nexus device running Android 5.1 or later to have access to WiFi Assistant. The feature will be supported in the US, Canada, Mexico, UK, and Nordic countries. To activate, go into your settings and find the Google menu. From there, open Networks and toggle WiFi Assistant on. This will allow you to connect to open hotspots with automatic VPN protection. There's also a toggle in the WiFi menu that turns WiFi Assistant on and off (use open WiFi automatically).
Google says this feature will roll out to phones over the coming weeks. I don't see it yet on my non-Fi Nexus 6P. There may be a Play Services or Connectivity Services update needed before this works.