Browse all the sub-reddits by group/title. This has all of them. Zoom in to see subcategories inside the major category. Zoom in more to see subreddits inside each subcategory. Wow!
Found in this conversation about how to find content on the internet. Search is often inadequate, because it's just a list of links. Better if we had a tree - a network chart - group links by category. That way, most categories you will not be interested in, so you don't have to look through those links.