BlueSky also started with ActivityPub but I believe they did something to their software to make it proprietary.
Friendly correction: BlueSky tried to use AP(ActivityPub) but they had problems with it (like user migrations). So they made their own protocol called the AT Protocol (Authenticated Transfer). ATProto is also an open protocol like AP and they are currently working to get it standardised: https://atproto.com/blog/kicking-off-the-atp-working-group
All software BlueSky uses is open source and self-hostable and already a bunch of different implementations have started to pop up independently. Sadly BlueSky still has the vast majority of users (like 90%) using their infrastructure.
While I don’t like BlueSky as a company, the software they are using is open.
Here is eurosky: https://portal.eurosky.tech/
Here is blacksky: https://blacksky.app/
Here is bookhive (a lot smaller): https://bookhive.social/
I found all of them by looking through the PDS index: https://atproto.at/pdses and selecting ones with enough users to signify open registration.
Here is a guide to self-hosting: https://atproto.com/guides/self-hosting#pds
All of these are links to PDS servers. These are the servers that contain all of your data. You can log into any ATProto apps using these accounts.