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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.
If you're using SICP in Scheme (or are you doing the JS version?) then you may want to look at How to Design Programs. It uses Racket which is a Scheme descendent so much of the language you've learned in SICP will work in it without issue. It also has a pretty good set of GUI and drawing capabilities you can find through the Racket docs page and will use some of with HTDP.
NOTE:
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Hence, a higher number means a more popular project.