Fediverse

Technically, anyone with the knowledge and interest can spin up a single-user ActivityPub server and go about their business, but generally these servers aren't being developed with that usage in mind. In other words, they can be overkill for individuals in terms of features or resource use.

That's where single-user software comes into play. Explicitly developed for individuals, or in some cases very small groups, to use, this software is lighter on resources and more focused in its features for individuals.

As to why you might want this: it enables you to benefit from many of the benefits of ActivityPub, connecting and engaging with others & building your own curated feeds, without some of the drawbacks of multi-user servers such as keeping up with federation/defederation decisions that may affect what you can interact with & follow.

So on to a couple lists, in no particular order:

"Microblogging" but with relaxed character limits:

1. GoToSocial
2. Hollo
3. Ktistec
4. Seppo - Note: more specialized/limited compared to above.
5. Takahe - More experimental compared to others above

Other

1. GoBlog - Simple blogging
2. Betula - Bookmark management & sharing.
3. Postmarks - Same idea as Betula, but some differences like ActivityPub commenting possible.

I'm sure there's way more, but these were a few that stuck out to me. Let me know any others you're into that I may have overlooked!