Modern and secure instant messaging
Some days ago, me and Valhalla played the game of trying to see what happens when one turns them all on: I would send her screenshots from my Conversations, and she would poke at her Prosody to try and turn things on:
Valhalla eventually managed to get all features activated, purely using packages from Jessie+Backports:
The result was a chat system in which I could see the same conversation history on my phone and on my laptop (with gajim)(https://gajim.org/), and have it synced even after a device has been offline,
I now have an XMPP setup which has all the features of the recent fancy chat systems, and on top of that it runs, client and server, on Free Software, which can be audited, it is federated and I can self-host my own server in my own VPS if I want to, with packages supported in Debian.
Valhalla has documented the whole procedure.
If you make a client for a protocol with lots of extension, do like Conversations and implement a status page with the features you'd like to have on the server, and little green indicators showing which are available: it is quite a good motivator for getting them all supported.