And of course more broadly, Matrix as a whole isn't even limited to only messaging apps, although that's where most of the focus has been thus far. There have been some fun demos/projects in the past though:
- Synchronized presentation slides: https://github.com/Half-Shot/matrix-presents
- MIDI over Matrix: https://github.com/McOmghall/midi-over-matrix
- VR videoconferencing: https://matrix.org/docs/projects/other/matrix-vr-demo
And of course, there is the Matrix spec itself, but I learned most of what I know idling in matrix channels (#matrix:matrix.org and #synapse:matrix.org) and hosting my own synapse instance.
FluffyChat is similar, and seems to be a bit further along (it has basic e2e crypto support for example) -https://christianpauly.gitlab.io/fluffychat-website/en/