Misskey. It's not exactly niche, but it has a lot of cool features.

For example:

• Drive features to reuse your image or other media.
• Misskey games, so far only have Bubble Game and Go.
• Groups
• Antenna, basically subscription for any post with your set of rules.
• Playful profile decoration, similiar to Discord/Steam.

NodeBB. It's a fairly popular webforum, but ActivityPub support is fairly new. It's really something else to see the Fediverse through a the lens of the old Internet.

https://codeberg.org/grunfink/snac2 and https://gotosocial.org/

The snac2 dev made it using only openssl and curl libraries. It doesn't even use a proper database, so it's very lightweight.

Gotosocial is basically mastodon but easier to set up on the server and lighter on resource usage.

@irelephant [he/him] late to the post but i have been loving hubzilla. Its quite different, offers features for hosting communities (posts, articles, photos, calendar, chatrooms, wiki, webpages, etc.) and has impressive privacy controls that i've not seen till date (except other softwares of this family like friendica, forte, streams).