Push Notifications in Android Platform
My understanding/experience with Android push notification are:
C2DMGCM - If your target android platform is 2.2+, then go for it. Just one catch, device users have to be always logged with a Google Account to get the messages.MQTT - Pub/Sub based approach, needs an active connection from device, may drain battery if not implemented sensibly.
Deacon - May not be good in a long run due to limited community support.
Edit: Added on November 25, 2013
GCM - Google says...
For pre-3.0 devices, this requires users to set up their Google account on their mobile devices. A Google account is not a requirement on devices running Android 4.0.4 or higher.*
(cross-posting from an answer I gave to a similar question - Does Android support near real time push notification? )
I recently started playing with MQTT http://mqtt.org for Android as a way of doing this sort of thing (i.e. push notification that is not SMS but data driven, almost immediate message delivery, not polling, etc.)
I have a blog post with background information on this in case it's helpful
http://dalelane.co.uk/blog/?p=938
(Note: MQTT is an IBM technology, and I should point out that I work for IBM.)
Android Cloud to Device Messaging Framework
Important: C2DM has been officially deprecated as of June 26, 2012. This means that C2DM has stopped accepting new users and quota requests. No new features will be added to C2DM. However, apps using C2DM will continue to work. Existing C2DM developers are encouraged to migrate to the new version of C2DM, called Google Cloud Messaging for Android (GCM). See the C2DM-to-GCM Migration document for more information. Developers must use GCM for new development.
Kindly check the following link:
http://developer.android.com/guide/google/gcm/index.html
Google's official answer is the Android Cloud to Device Messaging Framework (deprecated) Google Cloud Messaging(deprecated) Firebase Cloud Messaging
It will work on Android >= 2.2 (on phones that have the Play Store).