Quicksy is a spin off of the popular XMPP client Conversations with automatic contact discovery.
You sign up with your phone number and Quicksy will automatically—based on the phone numbers in your address book—suggest possible contacts to you.¹ Quicksy can be downloaded for free from Google Play Store. The source code is licensed under the GPLv3 and available on Github. Under the hood Quicksy is a full-fledged XMPP client that lets you communicate with any user on any publicly federating server. Likewise users on Quicksy can be contacted from the outside simply by adding +email@example.com to your contact list.
Aside from the contact sync the user interface is deliberately as close to Conversations as possible. This allows users to eventually migrate from Quicksy to Conversations without having to relearn how the app works.