Seamless patterns are a great option for backgrounds, textiles and merchandise. They usually come as image tiles which can be place side by side to create an infinite, repeatable design.
My seamless patterns come in vector format (.ai, .eps), as .pdfs and also as .png files—often with a transparent background option. If you have access to a vector editing program like Adobe Illustrator you can edit the colours and formations of the patterns. Or, if not, you can use them as they are.
Pattern files can be pulled directly into Illustrator and imported into your Photoshop libraries.