This site offers a searchable directory of lemmies
This Lemmy has scripts to make it automatically federated with all lemmy instances, so its /all feed is a global /all feed.
https://lemmy.directory/home/data_type/Post/listing_type/All/sort/New/page/1
Apart from the other ones people mentioned, you can search them on https://browse.feddit.de/. Happy lemming!
There's a few subs designed for that as well:
This is a great way to find servers, but I don't see how i can use it to find specific communities. Let's say I wanted to see if a community has been created that discusses the book series The Wheel of Time. Is there a good way to look for this, within all the servers?
You could search on https://browse.feddit.de - but it would depend what name had been given to the community (if one exists of course)
Had a look myself, and there is https://lemmy.ml/c/wheeloftime :-)
Here's a good place to start: https://sh.itjust.works/post/48334
