Matomo - Matomo is an open-source web analytics platform
Placemat - Pretty nice placeholder images.
Khagwal 3D Illustration Library - Free 3D illustration library for web & mobile app designers
PlaceKitten - Dead-easy placeholder images. Of kittens.
Three.js Journey - The best course to learn how to create stunning 3D websites
placeholder.pics - Lightest way to include placeholder images in your design