There are quite a few OSS Zanzibar-inspired authorization services/servers:

  - SpiceDB (https://github.com/authzed/spicedb)
  - Permify (https://github.com/Permify/permify)
  - Warrant (https://github.com/warrant-dev/warrant)
  - Ory Keto (https://github.com/ory/keto)