Open source Push Server

• PushSharp: [C#] :A server-side library for sending Push Notifications to iOS (iPhone/iPad APNS), OSX (APNS 10.8+) Android (C2DM and GCM - Google Cloud Message), Chrome (GCM) Windows Phone, Windows 8, Blackberry (PAP), and Amazon (ADM) devices!
• A good getting started guide with PushSharp
• PushSharp.Web: [C#] A web wrapper for PushSharp
• Original article behind this
• AeoroGear UnifiedPushSeerver: [JAVA]: Easy and promising - does have a Openshift gear and can be tested over Openshift
• aerogear-push-server-push-notifications-made-easy guid from Openshift
• Pushd:[Node.js] :Pushd is a pluggable "unified" push server for server-side notification to mobile native apps, web apps etc. With pushd you can send push notifications to any supported mobile platform, web app or HTTP server from a single entry point. Pushd takes care of which device is subscribed to which event and is designed to support an unlimited amount of subscribable events.
• Loopback.io: [Node.js]: Loopback is basically a nodejs framework to easily create api - but it does have component to create push servers.
• Node-Push-Server: [Node.js]: It looks like a stripped down version of Pushd.