SignatureSounds.org is a website run by a sound designer (Phil) that offers a huge library of high-quality, royalty-free sound sample packs, many of which are free under a CC0 license. You can browse and download field recordings, foley, ambient textures, percussion kits, and more. There's also a paid “Signature Soundbank” bundle that gives you access to 130+ sample packs, with new content added regularly, to help support the site.