iPad: If you do the majority of your online shopping at Amazon and you've got an iPad, the Windowshop app is an official app from Amazon—"a top to bottom rewrite of Amazon.com" designed specifically for the iPad.
The app is pretty self-explanatory, and after playing around with it for a bit, it feels really nice. When you first launch the app, you can navigate through categories by swiping right and left, and scan bestsellers within each category by swiping up and down. You can click a category title to dive into the category, and within that category the same rules apply. Instead of Electronics and Books categories, however, you're looking at sub-categories like Accessories and Camera & Photo.
You can, obviously, search, add items to you cart and buy them, add to your Wish List, read reviews, and do pretty much everything you can already do from the web site. The app just tailors it all to the iPad.
Windowshop is a free download for iPads only.