The Cutting Room Floor - Research on unused and cut video game content. Definitely one of my all time favorite sites to spend spare time on.
https://tcrf.net
100% agree! Unseen64 unearths some seriously cool stuff. I think an even equal pairing to unseen64, and in the same vein TCRF, is https://hiddenpalace.org/
What Unseen64 and HiddenPalace find and preserve is seriously invaluable in this area of interest.
Ken Fern's Plant databases https://theferns.info/
U2-3 - The Famous 1000 copies of U2's first ever release https://www.u2-3.com/
Dana Countryman's Virtual Museum of Unusual LP Cover Art (may contain some albeit low quality NSFW artwork) http://www.danacountryman.com/danacovers/danacovers.html
One Terabyte of Kilobyte Age - Digging through the Geocities Torrent https://blog.geocities.institute/
and finally, The End Of The Internet https://hmpg.net/
I've kept a bit of a running list on my site here: https://jason.nabein.me/#links