Here’s a list of 130: https://the-federation.info/#projects

One I like not mentioned so far is bookwyrm, which is like a federated goodreads (track books you’re reading, review them, read other’s reviews).

Edit: the flagship instance is https://bookwyrm.social

I think it’s worth noting that Mastodon is by far the most mature. Everything else is buggy and may not always work the way you might expect but I think many are still worth using if you can put up with the quirks.