I've always heard very positive comments regarding LensRental.com but I think their new "LensRental Keeper" program if brilliant! "Just rent an eligible item, and then we’ll give you a starting purchase price based on the age of the copy we send you. We’ll even give you a credit for a portion of the rental fees." I suspect this will shake up the second hand marketplace. Pretty exiting!!
(image directly linked from LensRental.com)
(image directly linked from LensRental.com)