After unveiling TestFlight beta testing at WWDC and rolling it out for internal testers a few weeks ago, Apple has today opened up TestFlight beta testing for everyone. From the Developer News website:
You can now invite up to 1,000 testers to beta test your iOS apps by simply sending them an email invitation through iTunes Connect. Once they accept your invitation, testers can install your beta app on their iOS devices, get updated builds, and provide feedback, all within the TestFlight app.
I tried the TestFlight app (released last month), and while it doesn't have all the features (from a tester's perspective) of Hockey, it's nicely integrated with iOS. You can receive push notifications for new beta builds, and beta apps installed from TestFlight get a special orange dot on the Home screen.