I used Google AdMob, there is some plugin on their site that supports Windows Phone apps. You will need to implement it in Visual Studio as there is no support for Unity
I'm currently using Reign Studios mobile pack (Reign) and it works fine with AdMob and PubCenter for WP8. It's not free, though.
I actually tested AdRotator just a couple of hours ago (AdRotator - The AdRotating solution for your apps & games) and it worked fine with AdMob, Adduplex and PubCenter. It' free and has support for other providers but I haven't tested them. AdRotator comes with a showstopping "bug" for me, though. Currently, your app has to display the ad on your first scene, otherwise it won't work.