All markdown files that you control. You can always sync it with Dropbox or any other if you choose to. Markdown files will be readable forever, and it is free.
Late to the party, but there's a "canvas mode" in Obsidian where you can sort of group notes together and drag them around. Other than that, it's also got a graph view, so provided you tag and link notes properly, it will show you related notes in the graph.