I have some (mostly game related) suggestions:
Abandonware:
https://www.myabandonware.com/ - Repository of abandoned PC games. A lot of old DOS era stuff but also a good number of more modern games. The list of games on abandonware sites has shrunk significantly in recent years as GOG has gained in popularity and companies realised they can sell 20 year old games to boomers. Still some gems to be found which are stuck in licensing hell or otherwise abandoned (No One Lives Forever, Alice, Heretic II and Chronicles of Riddick for example).
http://www.abandonia.com/ - similar to above and probably a lot of overlap between the two.
http://www.homeoftheunderdogs.net/ - I swear this used to have direct downloads years ago but now seems to just have links to google searches so less useful than the other two.
Emulation
https://vimm.net/ - ROMs/ISOs up to PS3.
https://cdromance.com/ - ROMs/ISOs. Lots of Japanese only games if you are into the weeb stuff. Has annoying pop-under ads, I usually prefer vimm.
Other
http://the-eye.eu/ - large archive site, a bit like archive.org. I mostly use it for games.
http://www.replacementdocs.com/news.php - archive of old videogame manuals.
https://videogamecritic.com/ - videogame review site which I'm pretty sure has been around since the 90s (and looks it). I think its literally one guy trying to review every game in existence for every console. Pretty simple - 1 screenshot,a short text blurb and a score. Nice to waste some time on or check if that NES rom you downloaded is worth playing.
https://www.ps2-home.com/forum/page/opl-game-compatibility-list-1 - a bit niche but if you have a soft modded PS2 some games don't work properly off the HDD. This site has a list of games and lists which version of OPL works, any tweaks needed to get it going etc.