Cal.com

Cal.com is built for developers who want control over their scheduling infrastructure. You can run it on your own servers or use their hosted version, and either way you get a full API and embeddable booking pages to integrate scheduling into your product. It's not just a booking page, it's a platform you own.

The open-source approach means no vendor lock-in and full visibility into how bookings work. Teams building products that need scheduling as a core component can customize and extend it without depending on a third party for every change.