Children oriented websites, especially for brands. If you liked a toy, series, even a food product chances are that their website had an add free (as the website was the add, let's be honest) space with free flash games, coloring pages to print, little promo videos or animations, etc.
The ones I miss the most were the Disney Jr, Barbie, and Paz the Penguin websites.
Children nowadays don't have safe and fun places to be on the internet, and those were formative for me. It's either social media (horrible for being social media), platforms like Roblox (bad moderation, very unsafe), or are being shoved the same adds we all suffer from in a very aggresive way.
I don't want to sound too much like a "THINK OF THE CHILDREN" nut, but there was a genuine sense of wonder and fun when I was a kid and on the Internet, and the spaces that made it that way are no more...