The Texture Vault is a source for royalty free textures and backgrounds. Great for those that need cool collage papers. Royalty free means that you can get these artist created backgrounds for free and use them in your work without paying a fee. These are still copyrighted works so they may be used in the same way that we use other published works (documenting the source and as part of a larger work of art).
Check out these from an artist I follow on Twitter.
What are your thoughts on using others' art in your work?