Subtlepatterns.com is a great free resource for high quality repeating backgrounds for use in your designs. It was created and is actively maintained by Atle Mo.

With subtlepatterns.com you can simply browse all of the backgrounds and when you find one that you like you can download the image. He even provides a 2x version for use with high density displays. If you are willing, you can even purchase his PhotoShop extension here for $11.99. It will automatically sync the images and will help streamline your design workflow.

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You can give Subtlepatterns a try here or download the PhotoShop extension here.