Integrated environment for building not only native Android but iOS and Windows apps too.
Ranked in these QuestionsQuestion Ranking
Pro Cross Platform
Xamarin is a platform on which you can build cross-platform mobile applications for Android, iOS and Windows Mobile and use only one codebase.
Pro Open source
Xamarin.Android and the whole Xamarin SDK is free and open source and released under the MIT license.
Con Lacking third-party library support
Having to use third-party libraries in Xamarin can be a real pain. Since Xamarin uses C# and third-party libraries are written in Java, you have to create bindings to use them in a Xamarin app. Which is cumbersome and wastes a lot of time. Especially considering that the Xamarin docs are not very good when it comes to this part.