Pattern8
Great collection of Seamless Patterns!
If your looking for good selection of patterns, check out the Pattern8.com site. There are lots of free seamless patterns to select from. These would make great computer backgrounds.
From John Rawsterne, the founder of Pattern8.com:
Pattern8 was created to make it easier to find high quality free seamless patterns. Our sister site, PatternHead.com, offers free vector and pixel patterns but it can be hard to find the pattern you need. Pattern8 keeps things simple and allows searching by color. You'll find lots of free pixel patterns here. When you preview a pattern you'll find a link to the vector version of the pattern from PatternHead.com (if available).
What's nice is that you can select a color that you want and then pick a pattern in that color. So if your background looks best in green, you can find all the green patterns that may work for your design. Since these are seamless, there's no limit to the size you need.
John's Been Busy...
John Rawsterne hasn't been active for a while, no new blog post since July 19, 2013, and the last twitter post was May 23, 2016. Who know's how long these sites will stay around. I understand that it's a side project so you update only when you have time to.
I think it's certainly worth checking out to get some design ideas for seamless background. The Santa Hats patterns are great!