Tucked away in some secret underground sweatshop are the hardworking scientists of Umbro Adidas et al beavering away making cutting-edge design shirts with all sorts of fancy material ( to try and justify the £45 price tag of course).
They can fill the promotion material with as much guff about space-age fabric as they like, but if they fans don't like they won't buy it, simple as. Therefore these leaks can be a good way of gauging the fans reaction to a shirt design.
Or it could just be the work experience kid illegally taking pictures of new kits with his phone and posting them online.
Anyway here is a few of the more interesting ones, both confirmed and unconfirmed, that I found online:
Chelsea home kit 2009/10