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Magma
Minecraft Forge Hybrid server implementing the Spigot/Bukkit API (Cauldron for 1.12) (by magmafoundation)
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WorkOS
The modern identity platform for B2B SaaS. The APIs are flexible and easy-to-use, supporting authentication, user identity, and complex enterprise features like SSO and SCIM provisioning.
Note2: Magma also works (again it's very unstable) https://magmafoundation.org/
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.