Subtle Patterns is a resource I reach for on almost all web based projects. It’s a simple yet powerful collection of, well, you guessed it, subtle patterns. The textures repeat perfectly, so are great as background images. There is a Photoshop plugin (paid), WordPress plugin, iPhone app (to test out that retina display), and also bulk downloads every now and again. There are over 300 patterns now available, and best of all, they’re free!
You can also contribute to the site with a pattern of your own. Check out the Italics Bold Submission, Old Moon.
And here are a few favourites.