Probably the best way to search for communities, though feddit.de had one earlier I think?
Masto/microblog specific: a.gup.pe though you probably know that one already dame.
Generally though, these services, especially the first two you list @dameoutlaw really emphasise the value that secondary or extra-platform platform services can bring.
I’d suspect, as an ecosystem architecture, leaning more into extra-platform services and reducing the expectation that every platform does everything would be a path to a more fruitful and sustainable fediverse.
Devs on the Fedi gotta find a way to work together better!