A personal favorite site of mine is https://crus.cc/ (https://crus.cc/)
I joined the cruelty squad community right before it was out of early access and I was quickly drawn to the modding community. De Sinople made the site and did a wonderful job! It is a catalog of a bunch of custom content that the modding community put out and is a great representation of how amazing they all are. I never got into making levels really, but I would occasionally make custom music for them and that was always really fun to hear a song I had made in an actual game.