Something obscure that's fascinating to me as a UX designer, are the many "fan redesigns" of major operating systems and applications like Windows, Chrome, media players, and so on.

They often have UX deficiencies, or don't scale to dynamic and more complex UIs, but that's not the interesting part - the interesting part is how you can infer from them how people are experiencing the "official" UIs, and what they find deficient about them.

It's like an accidental critical analysis of widespread software. Hugely valuable in understanding what makes UIs work or fail for people.