Push 4 Free is a very clean and simple push service as a server and client solution.
It is my answer to all the "free" push services (including GCM) where you eventually have to pay for each message send (!!).
You will need to install a P4F server to connect. This is only the client for receiving the notifications.
More information can be found on the github p4f site:
The whole protocol, android app and webservice are GPLv2.