5·1 year ago- LibRate (https://codeberg.org/mjh/LibRate), implement ActivityPub. Development in halt.
- Kitsu (https://github.com/hummingbird-me), specialized for anime and manga database. Not implementing ActivityPub sadly.
- Visual Novel Database (VNDB, https://vndb.org/) has open source their site: https://code.blicky.net/yorhel/vndb While its system is mainly focused for visual novel (like heavy tagging and character database), it has the most advance release version compared to any other kind of database. Might be useful for making version database of movies, books, or any other media. No decentralization.
- AniList (https://anilist.co/), anime and manga only with heavy social features. Has promise to open source their project. No ActivityPub yet, but has possibility.