I adore Society6. Endless gorgeous pieces of artwork to be printed on a myriad different items - there really is something (well, many things) for everyone!
The prices are very fair and I love the thought of some of my payment also going straight to the artist. It also looks quite simple to use from an artist's point of view - upload your design and away you go. The "social-media" aspect to the site is brilliant, allowing you to "promote" and comment on your favourite pieces, allowing the best designs to work their way to the top.
The items themselves are great quality, and the customer service, if ever needed, is rapid and pleasant too.
Highly recommended!