n8n

n8n is workflow automation in the Zapier mold, built to run on your own server: connect APIs, databases and services in a visual node editor, add code steps where clicks fall short, and keep the data flow entirely in-house.

Licensing is "fair-code" rather than pure open source — self-hosting for your own use is free, but read the terms if you plan to offer it as a service. The default SQLite storage is fine to start; busy instances move to Postgres.

The strongest option when your automations need real logic — branching, loops, error handling — not just trigger-action pairs.