If you're using GitHub as your central Git repository, you could take a look at Travis CI. It's a cloud-based CI system, so you don't have to manage the server yourself. I'm thinking of trying this out for some of my open-source projects.
If you want to stick with Apple's tools all the way, try out Bots. It ships as part of Xcode Server. I've been considering using this but haven't got around to setting it up yet. It's obviously very tightly integrated with Xcode and Git, which is fantastic if you're doing Apple-ony work, but as soon as you decide to branch out onto other platforms you'll find that you have to maintain two CI systems.
I've used TeamCity in the past. Compared to Jenkins it's very easy to set up and manage.
Lastly, you could take a look at Jenkins. From what I remember Jenkins is far more complex to install than it should be, but I admittedly didn't try terribly hard to get it working.
Up until fairly recently I ran my own CI system. I had a VirtualBox virtual machine running OSX Mountain Lion with an install of TeamCity. I encountered issues where the combination of Java (TeamCity and Jenkins both require it) and Xcode exhausted the virtualized RAM and caused the VM to completely lock up. If you have an abundance of RAM on the host machine that shouldn't be an issue, but it was a little annoying for me.