May I suggest Matrix/Riot? It's been posted on HN a few times and seems to be growing really nicely - I've gotten pretty into it over the past few days. Open source, federated, syncs between devices, interoperable with bridges to other protocols like IRC, web/Android/iOS clients, e2e encryption (similar algorithm to Signal) is being tested on web and is planned to arrive on mobile in a month or two. http://matrix.org/