I made a map of the various parts of Reddit, which I’m posting here in case anyone’s interested. The code was written in hacky Python using BeautifulSoup; the graph was created with graphviz and rendered with Inkscape. When producing the graph, I filtered out all of the subreddits that didn’t link to anyone else, and also I filtered out some other subreddits I didn’t like that were messing up the structure of the graph.

There seems to be one main group that includes all the big subreddits, and also some smaller, higher-quality ones; a cluster of Linux/open-source subreddits; and a cluster of music subreddits. More than that I can’t say; it’s likely that there exist other clusters isolated from the rest of reddit.