Jenkins + HockeyKit
An alternative is TestFlight:
https://testflightapp.com/
They offer a hosted solution, so it's not being hosted on your own server, however they provide a lot of features.
There is also a plugin for Jenkins that allows you to publish your builds to TestFlight:
https://wiki.jenkins-ci.org/display/JENKINS/Testflight+Plugin
Also HockeyApp:
http://hockeyapp.net/They provide a hosted solution using HockeyKit
If you are looking for an open source alternative to HockeyKit, check out Knappsack.
There is also a hosted solution with a free tier that might work for you. Here is a link to the source on GitHub: Knappsack Source
Currently, there isn't any way to integrate Knappsack with your build server, so a custom solution is needed there.