Does anyone know of a good Firebase alternative which is either (1) still independent or (2) run by a company with a reason to maintain it?
You can use Cloud SQL, Cloud Storage, Datastore, Compute Engine, App Engine, BigQuery, DNS, and all the APIs á la cart, and I'm sure the same will be true of Firebase.
In case you don't know, the APIs that used to be bundled with App Engine back when it was the only cloud product, like Datastore, have been unbundled over time. I assume remaining App Engine-only APIs like cron are under consideration for unbundling, but there's no precedence for an already unbundled API being bundled to App Engine.
https://parse.com/ - freemium
http://backendless.com/ - freemium
http://www.kinvey.com/ - freemium
http://deployd.com/ - open source
http://firehose.io/ - open source
http://www.baasbox.com/ - open source
https://www.meteor.com/ - open source
As for the open source solutions, I specifically want to pay someone money to solve this for us. We've been running ShareJS, but it's annoying to maintain and fix.
If you want to add realtime to an existing project that already has a database, then I suggest checking out Fanout ( https://fanout.io/ ). We intentionally don't want to own your whole stack, and we've opened our code and protocols so that you're never trapped. Would love to get your feedback.
http://www.socket.io (open source)
Usergrid enterprise-grade hosting and support provided by Apigee (as API BaaS): https://developers.apigee.com/get-started