At the Google IO 2012, the beta version of Android push notifications system called C2DM was replaced by Google Cloud Messaging (GCM). GCM has many new features over the existing system. Refer to this document for details of the differences and details about migrating your existing systems from C2DM to GCM.

To get started, download the Extras > Google Cloud Messaging for Android Library from the Android SDK.This library provides the jars to simplify the development on both the server side and client side.

From the Google API console page, create a new project and generate the API key. Refer this page for complete details.

Google provides the sample apps in the SDK by default, you may download a very basic version of them containing only the code discussed in this tutorial from here.
The same application is also available on play store here.

For .Net App download click here