Royal College of Art unveils annual secret postcard exhibition - Times Online
I had not heard of this event before. Reknown artists contribute a postcard size piece of their art with the proceeds going to the Royal College of Art's scholarship fund.
RCA Secret is an annual exhibition and sale of original postcard-sized art, made by professional artists, designers and illustrators, plus the Royal College of Art’s current postgraduate students. The postcards are signed only on the reverse, so the author of each work remains a secret until after purchase, and the signature on the back is revealed.
Here's the website where you can view all the art. I recommend that you clear a few hours before getting started!