Tweetbacks (we did the same name earlier today, entirely different product) is a gallery of Twitter backgrounds for you to download and utilize. You can also upload backgrounds if you have done something amazing, or even if it isn’t.
There was some cool ones to see and the site is ad driven on the right. Each background can have a rating, but since the site is new many didn’t have them yet. There is a link for the free customizable ones, but it was very limited in depth compared to the others we have (see the Profile Customization Tools category).
They do have a RSS feed to subscribe when new backgrounds are loaded into the site to keep you up to date also.