Google recently announced their new no-contract wireless service, Project Fi. Google is giving out instant invites for the next 24 hours! The Basic plan is $20/month and includes unlimited domestic talk and text, unlimited international texts and Wi-Fi tethering. Each GB of data is an extra $10, and you'll actually get any unused data credited back at the end of the month. With Project Fi you get international data roaming in 120+ countries without additional fees. Placing calls abroad will cost only 20 cent/minute. When connected to Wi-Fi the even lower Google Voice rates will apply making it a very interesting phone plan when traveling abroad. Project Fi automatically connects you to free, open Wi-Fi hotspots; when you're not on Wi-Fi, it moves you between its partner networks (Sprint and T-Mobile) for 4G LTE coverage.
One drawback is that currently only Nexus 6, 5X and 6P are supported.