https://fi.google.com/about/Google's creating their own cell service now that runs off of existing networks. What makes it different is that it uses whatever connection is strongest at your current location. At one point, it might use Wi-Fi, but then let's say you leave for work and AT&T's network is now the fastest, it would connect to that.
Also, it starts at $20/month +$10/GB, but whatever you don't use, you get back. I think it's 1¢/MB of unused data you get back.
Also, this is all without a contract.
Compared to European phone plans (I think, anyway), isn't this still kind of expensive?
Edit: it's only for Nexus 6 at the moment, sadly.