Dropbox had a real-time sync API for a few common data structures (lists & maps, I think), but then they killed it. Google Drive had the same thing for a while, but then they killed it. I'm pretty sure they were just CRDTs with solid hosting infrastructure and client libraries. Does anybody know of stable alternatives that are more resilient to some company deciding the project doesn't make enough money and ceasing development?

I second this. I am interested in any pointer. I think that CRDT like infrastructures will help FoC.

I haven't tried real-time sync yet so I don't know the latency profile, but <http://upspin.io|upspin.io> has been pretty good for my latest prototypes, and it has some excellent privacy and security characteristics. It doesn't natively support any data structures except files, though.

It look nice, but "Portions of Upspin may be made publicly accessible from the web [...], but there is no general web gateway for accessing Upspin content." And I am looking for technologies for the web plateform (like automerge, for example).