I noticed this wonderful tag cloud describing the interconnecton of topics on Math Stack Exchange. I recall seeing tools like this on the internet. I don't recall if Stack Exchange itself generates such charts.
As far as I know, this originated with some code by Piotr Migdal several years ago on Math Stack Exchange (with at least one update). The project was independent of Stack Exchange, and eventually was applied to other sites (e.g. Physics), becoming the fully interactive Tag Overflow, letting you generate your own graphs for various sites. There are also even bigger maps.