DBDiagram can help with drawing Entity-Relationship Diagrams painlessly.
Designed for developers and data analysts.

Open Source: No

Free: It has a free tier and paid ones.

My Take

I see to often suboptimal DB ERD. The key here is collaboration in early steps, share what you (the developer) think is the best schema and ask for a review.
I wrote about this similar tool: https://postgres.hashnode.dev/db-tools-use-sqldbm-to-model-your-db-online
I guess eventually people will use an open source free solution based on https://mermaid.js.org/.
Don't forget to document your reasoning, why you think this is the best solution, what are the keys of the table, when to de-normalize the schema...

DB Tools - DBDiagram is an online DB ERD