Soon, #DeltaChat enters the ring. It seems that they are using their internal peer-to-peer technology which is also used for their webxdc apps. This experimental feature is currently available in a branch on GitHub for Android.
To sum it up, it's great that we have all these alternatives.

I successfully made my own server for @delta! Woo hoo! Only using 215Mb RAM on a VPS so far. #DeltaChat #WhatsApp #Signal