Chat for distributed teams. Zulip combines the immediacy of real-time chat with an email threading model. With Zulip, you can catch up on important conversations while ignoring irrelevant ones.
Free for dev - list of software (SaaS, PaaS, IaaS, etc.) about about 21 hours ago:Zulip — Real-time chat with unique email-like threading model. Free plan includes 10,000 messages of search history and File storage up to 5 GB. Also it provides self-hostable open-source version.