As this seems to be a recurring topic, especially among newcomers, I am listing here the modules that I know for the Android platform. Keep in mind that to use these modules you will need to recompile the engine.
Admob - https://github.com/jlopezcur/GodotAdmob *
Admob - https://github.com/Shin-NiL/bbAdmob *
Appodeal - https://github.com/carlosmarti/GodotAppodeal *
Adbuddiz - https://bitbucket.org/reTTT/godotadbuddiz
Chartboost - https://bitbucket.org/reTTT/godotchartboost *
Chartboost - https://github.com/beocat/mamau_chartboost *
* Tested by me and working fine on Godot 2.1