For me one of the best kept secrets of Wikipedia is the Navboxes collapsed at the bottom of every article. They are very good for getting a bird's eye view of complicated topics and especially helpful for seeing how something fits within a complex hierarchy. Some of them are like minuature works of art that I'd want to hang up on my wall one day:

https://imgur.com/gallery/ILp6TtA

I get why they have to be collapsed by default, but I use a userjs to move them to the top of the page, uncollapsed, so I can use them to navigate (I set their CSS zoom to 0.3 so they don't take up too much space)