Earlier today, I had a chance to chat with Jeff Barr. Jeff evangelizes web services for Amazon.com and runs the web feed directory Syndic8.
It's remarkable what Amazon exposes through their web services. I'm particularly surprised by providing access to all of Amazon's customer reviews.
All of this has resulted in some clever applications. All Consuming, Amazon Lite, and Delicious Library are my favorites.
See also my earlier post, "May the best services win".