Cal.diy

Best for: developers who want Calendly-style scheduling running on their own server, with full control over the database and the booking data.

Cal.diy is the open-source community edition of Cal.com, a scheduling platform for booking meetings, consultations and appointments. It is built on the T3 stack — Next.js, Prisma, PostgreSQL and NextAuth — which makes it approachable to modify if you already work in TypeScript.

What makes it useful: self-hosting means client booking data stays on infrastructure you control, which matters for consultants handling confidential engagements or anyone operating under data-residency requirements. The stack is conventional enough that customising availability rules or booking flows is ordinary application work.

Where it falls short: the maintainers are explicit that Cal.diy is intended for personal, non-production use and should be treated as use-at-your-own-risk. Self-hosting demands real competence in server administration, database management and securing sensitive data. If you need commercial or enterprise-grade scheduling, the project directs you to hosted Cal.com instead — this is not a drop-in replacement for it.

Details: TypeScript, MIT licensed. Requires PostgreSQL.