This service is in beta and currently it has collection of around 8000 cover designs. Covers are classified into categories of Creative, Mood, For Girls, For Guys, Nature, Music, Sports, Entertainment. General. You can select a cover and add that as your facebook cover directly from website.
Soon companies can also join and have facebook covers designed for products and services. People who really like a product or service can endorse those facebook cover. It will definitely act as visual advertisement and it is free. Companies or media agencies can ever reach out to people who are influential and have large following to put a products/services’ facebook cover.
Update on 17/01/2012
There is one more website for facebook covers. Checkout http://fbprofilecovers.com