Not so much an out-of-the-box tool, but I am using OpenAI's API to generate complete knowledge graphs for my own tool: Contextualise (a topic maps-based PKM tool). Although not perfect, this approach is working suprisingly well.

Yeah, I also don't imagine using the raw output directly that much, but realize there must be many things one could do with the help of the new capabilities.

I've been thinking about things like automatically generating Anki flash cards for content I want to learn, and similar.

I am careful not to use it for actual learning - the information is not accurate enough to trust. But it can build a roadmap of details for you to research independently when you are new to a topic -- to fill in the "don't know what I don't know" level of knowledge. You can then independently research your knowledge gaps once they are identified.