Non-browser alternatives for all the things listed above websocket: some application-layer protocol running on top of TCP.

Non-browser alternatives for websocket: TCP

Specifically, you're looking for messaging protocols like AMQP, MQTT, XMPP, STOMP, or even HTTP because there are other HTTP clients than just the ones inside browsers. All these protocols describe some aspects of framing and semantics, and predictably, you need both a protocol-client and a protocol-server to run them.

Stuff like SNS and GCM is Messaging-as-a-Service, where they host the server, and give you a client to integrate into your code.