• There’s a multi-instance directory of communities at https://browse.feddit.de/. Though communities that JUST got created may take some while to show up there… and it seems like a moderately large number of communities have been created over the last day or two.
• Another strategy is just to browse through community list at https://lemmy.ml/communities/. Being the biggest instance, the users there have subscribed to the bigger communities on any instances that federate with lemmy.ml so that’s a good way to find popular communities.

In the very top you can click communities. If you then click all you’ll see all communities that your server knows about.

there’s also this which aggregates a bunch of instances:

https://browse.feddit.de/