DeltaChat is a messenger that uses email as the transport medium, so potentially almost everybody already has an account for it. Today, a they announced a new release, not only for the existing Android app but also for Desktop (Mac and Linux only so far) and iOS.
I think that is a very interesting project since it does not require yet another central server. It also supports encryption using the Autocrypt Level 1 standard, if both ends use DeltaChat. It is possible to use the same protocol (but without encryption) between DeltaChat and a regular email program.
In my tests with the older Android release it worked well. I created a new email address specifically for DeltaChat. That’s what I would suggest to everybody who wants to use it. I plan to replace Google Hangouts with DeltaChat when Hangouts gets shut down later this year. But I would need a Windows client first.