(I should disclose upfront that I'm the founder of FusionAuth)

The core of FusionAuth isn't open source, but a good portion of the code built around the core is open source. It is free and has a large, growing community and integrates with any language.

Plus, you get the benefit of a having a company behind it that is constantly improving and updating the product.

I'd be happy to talk more about our licensing model and why we chose it if anyone is interested.

Is the self-hosted solution limited in some sense or feature complete?

It has all of the same features as the Cloud version, including unlimited Enterprise Connections (SAML and OIDC) for free.

The only features that required a paid-Edition are: Connectors (connecting to LDAP and other backends), Advanced Registration Forms, and Breached Password Detection.

We are adding additional paid features as well, but our Developer Edition is reasonably priced. $125/month to start and that gets you up to 10,000 MAU. You can also test drive Developer Edition free for 14-days.

I have used FusionAuth for a few projects now, both personal side projects and work projects. My only qualm is that it is Java based and relatively heavy on the laptop if you’re running a local dev instance in Docker. I love the product though, the support is fantastic and the docs are straightforward. I’m a big fan, great work FusionAuth team.

Not open source, but free as in beer: https://curity.io/product/community/

It's a complex server, but if you need to connect to lots of third party systems it may be a good choice as it has support for all sorts of things and its plugins are open-source (e.g. BitBucket plugin: https://github.com/curityio/bitbucket-authenticator)

Yup, Keycloak. It's certainly a batteries included solution but my experience so far has been quite positive. Apps deployed have been for small provincial govt ones with individual app user bases varying from 1 or 2 users to 30k (size of the provincial public service). Keycloak does the OAuth2/OIDC work using the govs active directory for the identity provider. Keeping my eye on Ory as a perhaps more ala carte solution. edit: added Ory comment

The ORY stack: https://www.ory.sh

Thanks! Can you recommend any C++ client example to use the ORY stack?

The ORY SDKs are auto-generated using OpenAPI, so you could probably request one or generate one yourself (https://github.com/ory/sdk). I just put in a request for a Rust SDK and was able to auto-generate one locally for testing purposes pretty easily.

Any idea when will an official Rust SDK exist? Very interested. A link with a tracking issue / PR would also be good to have.

I'm currently investigating this area and have been recommended Parse and Supabase

Looks like C++ client support is still a request? https://github.com/supabase/supabase/discussions/5

Does it not have a plain REST API you can use? I've not looked into it in detail yet