The Hypothetical Library is one of the coolest things online I have found so far.
The concept is simple: Charlie Orr, a designer, asks real writers "to provide flap copy for a book that they haven’t, won’t, but in theory could, write, and then I design a cover for it."
Brilliant idea--and it pans out really well. Go check it out! A lust after books that unfortunately do not (yet) exist.