We have highlighted numerous Twitter background tools (see the Profile Customization category), Twitlay follows right along allowing you to upload an image, add text and instantly make it your background.
Of course, they will also design a custom background for $50, but you are after the free ones first. Categories run down the right, but you haw to fight your way through the ads to find much of anything, including the background creator at the bottom. Which is really a Google ad that blends in.
If they streamlined the ad placement and cleaned up the busy UI, it would be another place to point you for backgrounds.