Heppo.se is a Swedish ecommerce website (unfortunately, there isn’t yet an English version) with a sleek, edgy layout: though there are many reasons to be fond of this website, the motivation for having it showcased on Web & Patterns is its add-to-cart button.
When you click on it, a bright, trasparent rectangle appears above it and moves quickly over the page towards the cart area (top right corner), shrinking and fading.
This is obviously a minor feature, but it really shows a deep attention and care about the design of an effective user experience.