Cactus Comments is a federated comment system for the open web built on Matrix. It is GPLv3 licensed, written in Elm, and we host a public backend that's free (as in beer) to use.

Check it out and play with the demo at: https://cactus.chat about this event: https://bornhack.dk/bornhack-2021/program/cactus-comments-web-comments-built-on-matrix/