Webxdc brings web apps to messenger chats, in a simple file format containing HTML5, CSS, JavaScript and other asset files. All authentication, identity management, social discovery and message transport is outsourced to the host messenger which runs a webxdc app container file and relays application update messages between app users, letting each app inherit offline-first and end-to-end encryption capabilities implemented by the hosting messenger.
Directory of apps: https://webxdc.org/apps/
I don't know if any of these could be used in a free-standing way or not, but it might be interesting to try.
This repository contains a blacklist.txt of XMPP domains that are used by spammers and do not react to abuse complaints. Servers are added and removed according to the following rules. The track record leading to addition or removal is documented in the respective git commit.