Google Cloud Messaging for Android 

(GCM) is a service that allows you 
to send data from your server to 
your users' Android-powered device,
and also to receive messages 
from devices on the same 
connection. 
The GCM service handles all 
aspects of queueing of messages 
and delivery to the target 
Android application running on 
the target device. 
GCM is completely free no matter 
how big your messaging needs are,
and there are no quotas.