Misskey. It's second most used software in fediverse. Used in misskey.io, which has 10 thousands daily active user (possibly 100K-200K MAU).
Developed since 2014. Originally function as self-hosted microblogging, now turns into unique social media. For example:
- Misskey-flavoured Markdown, example
- social games (only two so far: Reversi and Bubble Game)
- emoji reaction like Discord (they're the first one to implement it on fediverse)
- Antennas (tracking post with any keyword)
- Pages
- Channels (groups)
- Clips (bookmark with multiple groups, kinda like Facebook bookmarks)
- Achievement
- optional ads banner (just in case the server admin wants to do community ads, usually used for indie games, comics, vtuber, or IRL art event)
Their community is mainly Japanese, they desperately need English contributor and community to help them grow.