There's also SimpleX chat and Briar, but I've used both of those less than Matrix. They seem to be aiming to solve the last few issues that Matrix has, like usernames and metadata leakage.

I consider Matrix to be closer to an "Enterprise" solution, like what a business or government or non-profit would use for secure communications (literally both French and German governments use Matrix), while SimpleX/Briar seem much more aimed at individuals just wanting control over their personal conversations.