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
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.syncano.com (freemium)
http://www.pubnub.com (freemium)
http://www.socket.io (open source)