Description
Initially, I was going to say put this as a Discord frontend because it connects to Discord through libpurple (my knowledge on how it works is none, as I've only just discovered this recently.) but I do not know if it could potentially fit somewhere else in the list.
Anyways, https://pidgin.im is a collective app that uses the libpurple library for users. It can connect to more apps and software than https://github.com/beeper can. What's great too is that it has a n official list of plugins, with both official and community made plugins being on there. It is however only accessible through desktop, as it has no mobile client.
I don't know if this counts as a frontend or not, but I figured since it also includes so many other chat apps into itself, (Matrix, XMPP to name a few) that it was well worth mentioning.