Twitter, completely skinned down to just the content and no login nags:
Nitter - https://nitter.net - https://github.com/zedeus/nitter
Youtube, also very skinned down and you can also collapse recommendations by default etc.:
Piped - https://piped.video - https://github.com/TeamPiped/Piped
Invidious - https://invidious.io - https://github.com/iv-org/invidious/
Stackoverflow, also completely skinned down to just the content and nothing more:
AnonymousOverflow - https://code.whatever.social - https://github.com/httpjamesm/AnonymousOverflow
For all of these I use LibRedirect to redirect the original pages to the open source/privacy friendly frontends: