Pidgin

Pidgin is a multi-protocol IM client for Windows and *nix desktops (see Adium for Mac). Pidgin 2.x series is woefully crap in many technical ways, but mostly does what it says on the tin. Development on 2.x has been in maintenance mode since about 2011, and most dev effort (if any) is focused on the 3.0 branch, which is pretty much a ground-up rewrite, and still hasn't been released.

Missing XMPP XEPs

XMPP support in Pidgin has been slow to adopt new shiny XMPP extensions (XEP). Tickets exist for them:

XEP-0280: Message Carbons
XEP-0313: Message Archive Management
XEP-0384: OMEMO encryption - Whisper Systems Axolotl over XEP-0163: Personal Eventing Protocol.

This is compounded by the XMPP Standards Foundation, which has dwindled of late to about two people who actually bother to turn up to six monthly meetings, who seem to talk about anything other than progressing any fucking standards beyond "experimental". Meanwhile, here are instructions on how to fix them.

XEP-0280: Message Carbons

There's now a Pidgin 2.x plugin.

XEP-0313: Message Archive Management

This is the bit that lets the server stash messages sent to you while you're offline, and the client to sync the history when next online, also like Hangouts. Especially useful for multiuser chatrooms. A patch potentially exists in the meantime. Pidgin 2.x is limited in its ability to store information on disk (unencrypted plain text, pretty much) which precludes implementing MAM without also implementing SQLite databases and so on.

XEP-384: OMEMO Encryption

OMEMO is now an XEP marked experimental, and there's now a Pidgin 2.x plugin.

Facebook, Hangouts, and Steam

These protocols used to be XMPP-based, but because nobody seems to understand how to implement it properly, and the XEP standards body can't be bothered progressing draft standards that have been in use for years, they have been replaced by proprietary protocols, and there are now plugins for them for libpurple:

Facebook Messenger protocol, based on a 2015 Google Summer of Code project.
Google Hangouts, protocol, also based on 2015 GSoC.
Steam, based on Valve's Open Steamworks API.

These just involve running make and lobbing the .so into your ~/.purple/plugins directory; see their respective README files for installation instructions. You will need Google protobuffers and GLib JSON libraries too:

sudo apt-get install libprotobuf-c1 libjson-glib-1.0-0

Discord, Mattermost, RocketChat...

Eion Robb has been churning out libpurples for all sorts of other protocols too - have a look at the list of third party plugins on the Pidgin wiki.