Can I use the sprites/models/textures/sounds on this site in my own projects?
That depends. Is your project commercial in any way (as in, do you stand to make money from it by selling it, advertising in or around it, including in-app purchases in games, or in any other way)? If so, then no, you can't. These resources are all copyrighted works and as such, cannot be used in any commercial project without express written consent from their copyright holders. We do not own and cannot license any of them to you.
That said, if your project is 100% free (e.g. a free fan-game or non-monetized YouTube video - basically any project that was made just for the sake of making it and will not be used in any way to make you money), use away! This is one of the primary reasons we collected all of these resources and we'd love to see what you do with them!
As a special side note, we'd also caution against using anything from these sites in games published to any established market place (like Apple's App Store or Google Play), even if they're completely free and contain no ads or in-app purchases. Because it's a formal publication at that point, it's still likely not allowed.