[Karrot_Kream]

A much deeper version of the tool using more systems dynamics features is Insight Maker[0]. They're a tool to actually build and simulate real life models in. The UX is a bit more confusing, but the tool is much richer.

[0]: https://insightmaker.com/

[jacques_chester]

Slightly annoyingly, Insight Maker supports the core idea of stocks and flows but doesn't really use the stock-and-flow notation.