I’m a serial petsite user >.< Of all the websites already mentioned here, I’ve held an account for most of them. I dropped them over time, because of the micromanagement though. My favourite are breeding sim petsites.

Of the places not mentioned yet, I’ve had an Acitius account for years, Furry-Paws, Leporidae (rabbit breeding sim), Cyska Siberians, Mycena Cave (rp/writing based petsite), Psypets, and there was another name I forgot— you collect mythical creatures/urban legend creatures (like the Loch Ness or Jackalope) by visiting different places on a map. That game was nice because it was a petsite where you didn’t have to log in frequently, just whenever you wanted to explore the map and learn some folklore.