My favorite supplemental materials has always been the official websites that J.K. Rowling created for her fans, with the earliest a defunct website released in 2000 at the announcement of the first movie. The first website was very much an early 2000s Pottermore, and had a sorting hat quiz, a wand ceremony, and many interactive games. I don't believe it saved your account back then, or at least I didn't know how to do so, but I remember always being sorted into Gryffindor except for when I messed around with other answers to see how to get different results. I also have a dim but strangely vivid memory that my wand from that time was the very same wand I ended up receiving both in Pottermore, and again when the switch was made to Wizarding World: A Holly wand, with Dragon heartstring core! Perhaps not so strange if we consider that J.K. Rowling created and wrote the tests very intentionally, but each quiz was quite different and yet gave me this same result very persistently, as though trying to tell me something!

I think that the Ceremonies that allow us to engage with this world, which were created and legitimized by J.K. Rowling herself, are such a special facet of the Harry Potter fandom and its development in a time when cyber and web driven content, or "born digital storytelling" is blossoming into its own. To have a creator who didn't grow up during the internet age embrace her fans who did, and whom were growing with Harry and wanting their own place at Hogwarts, is incredibly special.