There are several services available for testing mobile apps in the cloud on real devices:

Sauce Labs

Sauce Labs is the veteran and was co-founded by Jason Huggins, the original creator of Selenium.  Sauce Labs also works on development of Appium.

BrowserStack

BrowserStack is an economical choice for testing in the cloud, but while the offer “Live” mobile devices (manual testing only), it does not appear that they offer devices for test automation.

Xamarin Test Cloud

Xamarin is a pretty good contender, but focuses their tooling around Microsoft .NET / Visual Studio developers.  It does not appear that Appium is supported (directly) Still, they have a big selection of devices for those willing to stick to Microsoft tooling. It is also nice that their metering is a soft limit.

Perfecto Mobile

Perfecto Mobile has been around for a while as a service for Enterprises.  It is proprietary, but also supports Appium.  It comes with an Enterprise level price though.  You have to call and talk to a salesperson for any plan beyond proof of concept.

Keynote Mobile Testing

Keynote Mobile Testing used to be called Device Anywhere and their experience dates back to the days before smart phones.  I don’t have recent experience with them, but they used to offer only pixel based automation.  It appears they support Appium now.  But automation is not supported outside their enterprise sales channel.

Amazon Device Farm

Amazon Device Farm is the most recent entry in the field.  But it is not only quite expensive, but requires a lot of do-it-yourself.  Perhaps it will be more practical once tooling for recipes becomes available.

Price Comparision of Sauce Labs, Xamarin Test Cloud, Perfecto Mobile, & Amazon Device Cloud

* 25% annual discount available

** 15% annual discount available