Some very good answers in this post: I’m aware of kbin, mastodon, and lemmy. Are there any other federated applications out there? : nostupidquestions, some random copy&paste:

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

this diagram

From What other kinds of servers are on the Fediverse? | Fedi.Tips – An Unofficial Guide to Mastodon and the Fediverse:

As well as Mastodon, there are lots of other server types on the Fediverse, for example:

• BookWyrm – A social reading platform, an alternative to Amazon’s GoodReads

• Friendica – A general social network with no character limits, sort of like Facebook used to be on its older interface

• Funkwhale – Music and podcast storage and sharing

• Kbin – Link aggregation and discussion, sort of like Reddit

• Mobilizon – Event organisation, the Fedi’s alternative to Facebook Events

• OwnCast – Video livestreaming with a chat window at the side, very much in the style of Twitch

• PeerTube – YouTube-style video sharing site which uses P2P technology to allow even small servers to have videos go viral, as the more people view a video the more bandwidth it gets

• PixelFed – Photo sharing site, similar in style to Instagram and Tumblr

• WordPress – Independently hosted WordPress blogs can be turned into Fediverse servers using a special plugin

• WriteFreely – Minimalist blog where focus is on the text, like a calmer version of Medium

…and that’s just some of them! If you’re a techy person, you can see more comprehensive lists of server types at FediDB and delightful fediverse apps.

It’s not guaranteed that every federated app integrates with every other federated app in a particularly useful way. You kind of need to take it on an app by app basis:

• Kbin and lemmy integrate very well. “Magazines” on kbin show up as communities in lemmy. You have almost certainly already read and responded to posts and comments from kbin users, and you may have subscribed to communities on kbin.social, the largest kbin instance. Interacting between the two is pretty much seamless.
• Mastodon and Lemmy integrate, but less completely. If you’ve seen a post full of #hashtags and with an @thecommunity@instance mention, that’s probably from a mastodon user. I’m not sure how a lemmy user can initiate contact with a mastodon user, but a mastodon user can at-mention a lemmy community as if it were a mastodon user and doing that will create a lemmy post. Comments on the lemmy post look like replies to the mastodon toot.

Other fediverse projects will interact in varying specific ways, and you need to figure each pair out individually.

Try fedi.tips

From Lemmy you can’t use Mastodon (though you can the other way around - from Mastodon to Lemmy, but it’s suboptimal). Matrix follows similar ideas as Fediverse, but isn’t part of it, there’s no way of interacting with it. Kbin is similar to Lemmy and you can actually interact with it - visit for example !AskKbin@kbin.social and interact with the community. You won’t notice any difference, but you’re interacting with Kbin community (on Kbin you’ll find the community on https://kbin.social/m/AskKbin).

I made a list of Fediverse alternatives that may be helpful.

Some basic tips on using the ActivityPub services (Matrix is its own beast and stands slightly apart from them but you can add your Matrix ID to the settings on Lemmy and it gives you a button for secure messaging)

• Most will use markdown for formatting, as is used here.
• In quite a few services you can use the @ address format to follow other accounts - on the microblogging services you can usually also use the same format to follow communities in the threadiverse (Lemmy/kbin). You can’t currently follow individuals on Lemmy (you can on kbin) but it is a much requested feature.
• You can often reply on other services using an account on a different service.

Any other questions?