Active Matrix Rooms maintains a live index of public Matrix rooms across the federation. The search engine matches your query against each room's name, alias, topic, and homeserver domain. Results are ranked by messages per hour rather than by total members, which can be inflated by abandoned accounts.

I built this and it's been live for over 3 months stable at this point, used every day by people to help find the rooms that have active users. I created it after joining several rooms that didn't have anyone active, and I was seeing many other users do the same. It should never be the case that you need to join blindly to know whether there are messages being sent in it.

https://activematrixrooms.com/

If you have any suggestions, issues, or feedback please let me know, thanks.