The 2000s Cartoon Network US site and Nitrome ( still alive but feels like a shell of its former glory ).
I know I can still play a lot of the old CN flash games on Flash Point, but it's not as aesthetically satisfying as the old CN site.
As for Nitrome, I got a zip file that contained a lot of the games since they ain't available on Flash Point and I don't wanna use the modern site. The old games like Mutiny or IceBreakers are still kinda fun, even if games like Rubble Trouble, for some reason, don't run well on my potato desktop under Ruffle.
Those were 2 of my favorite game sites in the 2000s, before I learned about NotDoppler.
Edit:
After looking at various comments, gonna say I remember the old Pencilmation series back when there was maybe a few different shorts on their website. Back when evil blue pen man was the big bad. Before they, or whatever copycat it was, started making tons of them on yt that are nothing but mass produced slop. Lived long enough to become a villain.
I also remembered an old PopTarts website with some dumb flash game about going down the red carpet without getting toasted and/or eaten ( IIRC ) that lead to me finding Pencilmation.