Most mobile platforms have only two things in common: WebKit and a fast Javascript VM. Writing HTML(5)/Javascript code is the way to create mobile apps without rewriting them from scratch for every platform. To make these apps available in the various app stores, it's necessary to package them as native apps.
PhoneGap is one tool that implements exactly that for HTML/Javascript applications; Rhodes is another framework which allows to use Ruby in addition to Javascript to write application logic.
However, these frameworks still mean the developer has one big yak to shave: installing the SDKs of all the mobile platforms, setting up build processes, then getting the CI system to run the builds. That last step isn't as trivial as it sounds: iOS apps must be built with the iOS SDK - which only exists on the Mac.
The solution is to let someone else worry about the details - build services for mobile apps do just that.
The oldest entry in the list is RhoHub for users of the Rhodes framework. As a matter of fact, RhoHub provides more than just building and packaging. The code is pushed to the service via Git (RhoHub can host the repositories), where the build processes for Rhodes' supported platforms are started.
RhoHub also takes care of hosting data for Rhomobile's data synchronization service RhoSync. The Rhodes framework is MIT licensed, RhoSync and RhoHub are commercial.
Apparat.io is a new build service by uxebu that takes the app source and turns it into an installable application. The service is in a private beta right now, following and reading the @apparatio Twitter account is one way of getting an account at the moment.
A preview of the Apparat.io website and documentation is available, and shows that building for Android and iOS are currently supported. It's possible to point Apparat.io at a GitHub repository and have it build the application stored there. According to a tweet on Apparat.io Twitter account, Apparat.io uses PhoneGap to build the native applications for Android and iOS.
Another build service comes has been announced by the makers of PhoneGap: PhoneGap/build. At the moment, there's only a website that allows to sign up for the beta and to be notified when the service goes live.
Have you tried Rhodes or PhoneGap to build applications?