Fediverse Observer is awesome!
Btw, welcome to town @observer.
On the website https://fediverse.observer/ you can find any Fediverse instance, filtered/ordered by software, location, name, uptime, open signups & users.
The website prominently suggests these software projects as filters:
In the dropdown menu there are more options:
But that’s not even all. E.g. I saw that you can also see all Ecko instances here.
I wish this site was more well known that joinmastodon.org.