Stop Using Discord, Signal, Telegram &c. 
XMPP is a free (as in freedom) instant messaging protocol that has been around since 1999.
Since XMPP is a protocol like IRC or e-mail and is not a centralised service like Discord or Telegram you cannot be shut down and scattered by a central authority such as a group of admins, or some silicon valley company.
It is secure, open and is available through a wide variety of services that do not require personal information. You could even set up your own server.
XMPP features encryption in 1-to-1 conversations, with experimental encryption in multi-user-chats (MUCs), file transfer via HTTP (or external site such as 0x0.st or lainsafe) and audio/visual calls among others. "eXtensible" is the "X" in "XMPP" so it is richly developed and deliberately so. Along with a suitable VOIP substitute it can serve in a full replacement to bad IM services like Discord, WhatsApp, or Telegram.
A friend of mine wrote a couple of great articles about Discord and why you should stop using it here and later here, and the Spyware Watchdog also has an article about it here for those more concerned with technical aspects of privacy.
Services
Reputable services include Hot-Chilli/jabb3r, yaxim, NixNet, and YDFS/yourdata.forsale.
Clients
Many clients are available, this being one of the perks of an open standard. If you want one that just werks™ then conversations is a pretty good start, although to get it for free you have to use the F-Droid repo. It has numerous forks, one of which is the blabber.im app, available on F-Droid and the Play Store. It has added features and is generally a nice improvement over conversations.
Note, December 2023: I have fixed some dead links but I do not have the time to go over this section again. It may be outdated since I see the last time I updated this page was in the July of 2021. I still use Psi+ but I hear Dino is fine. Digdeeper has a more recent page on this and I trust him, so go there if you want more detail.
For desktop use, you have a few choices:
- Gajim
- Only desktop client I've fount suitable for all-round usage. It has no actual problems with anything and is totally solid, however it is written in python so it is somewhat slow and I don't like the way GTK3 looks personally. Its the only one I've used on Windows myself. Use this one if you're not sure.
- Psi+
- Psi/Psi+ is an excellent client that has a problem with encryption, specifically what happens with sending encrypted messages when someone is offline: they won't recieve it. If you're not using encryption often or don't care, this is the best client. I recommend building it with the flag DCHAT_TYPE=BASIC in order to prevent it using QTWebEngine to render messages. Make sure to download the plugins as well, so you can use OMEMO and the image previewer.
- Dino
- Dino is a just werks-type client that is unfortunately afflicted with the dreaded curse of the foot. It's GTK3, only recently added simple features like room invites and has a total lack of customizability. It does just work though. Not available for windows.
- Profanity
- Profanity is a pretty slick console client. Takes a bit of getting used to, especially with regards to OMEMO, and uses GNU Readline for its input. I've not tried its main alternative, Poezio, but I recommend this for text mode usage.
