Google Zanzibar is an authorization system that manages permissions at scale, offering a relationship-based model for complex permission scenarios. Several open-source projects have implemented its core principles, providing powerful options for building scalable authorization systems. The top five open-source implementations of Google Zanzibar in 2024 include SpiceDB, Permify, OpenFGA, Warrant, and Ory Keto. These solutions vary in their focus on large-scale applications, multi-tenant scenarios, developer experience, microservice architectures, and managed services. When selecting an FGA implementation, consider factors such as scale, consistency, resources, and integration needs to align with your organization's specific requirements.