Google has opened invite requests for Project Fi, a new network service being rolled out in partnership with T-Mobile and Sprint that will make your calls and texts transfer seamlessly from cell networks to WiFi as you move. It works with any device that supports Google Hangouts. You can keep your phone number when you switch to Project Fi. Plans start from $30 per month.
Of note, the Google Nexus 6 is the first phone to be compatible with this network.