Mermaid is a javaScript-based diagramming tool that renders diagrams from text definitions. It handles text-based, markdown, git-friendly, and many chart types, and it's best suited for developers wanting text-based diagrams in markdown. It has been around since 2014, giving it a 12-year head start in building out integrations and refining the product.

The core product is entirely free. Since it's open-source, you can self-host for free with no user limits. It's aimed at individual users, so it's fast to set up but may lack team-management features if you scale.