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.