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)

Worth mentioning Casbin as well (https://github.com/casbin/casbin) - it's been around for a while and takes a slightly different approach. Instead of being purely Zanzibar-inspired, it uses a PERM (Policy, Effect, Request, Matchers) metamodel that lets you implement RBAC, ABAC, or ReBAC depending on what fits your use case.