Only a few years ago, most of these web services mentioned below could not be imagined. Mashups, API's, and web services are taking over and just plain making it easier to get things done. I've listed some of my favorites below...
Amazon Fulfillment Web Service
Amazon FWC is one of the most exciting services I have seen lately. A web service which literally intersects with the real world. If you are an eCommerce merchant and don't want to dive into the routine hassle of handling fulfillment of your orders, Amazon will do it for you.
Send them your physical inventory, Amazon stores your inventory, customers purchase your products, Amazon picks and packs your inventory and ships products to your customers. This can be a breath of fresh air for eCommerce store owners. Imagine a customer purchasing several of your products through your eCommerce store. The products show up at the customers door with no interaction from you. The process is nearly 100% automated.
From the Amazon FWC web site: Allow merchants to "tap in to Amazons network of fulfillment centers and (its) expertise in logistics" to simplify the distribution of goods sold through their storefronts.
http://aws.amazon.com/Lulu
37signals uses it so it must be good. Lulu is free self-publishing - a service for budding authors. Setup an account, set the price of your material, and they print and sell your book(s) for you. They keep track and pay out royalties. There's no need to send your newly published novels to Amazon FWC as Lulu handles fulfillment services for you. Ask around and you'll find the service is mainly focused on novels.
Anyone can publish and sell print-on-demand books and e-books, online music and images, custom calendars and books. Publish, create, and sell your own books, manuals, guides, etc. Using this service can help authors get started on their own quickly and easily. They even promote an ISBN service where you can purchase an ISBN for your material and painlessly get your book listed on all major online bookstores, including Amazon.
http://www.lulu.comYouTube (Free Video Hosting)
Do you have any digital videos you'd like to quickly share with friends and family online but don't want to deal with setting up hosting, paying for bandwidth, usage limitations, and other hosting issues?
Create a free YouTube account and start uploading your videos. Just sign up, create an account, and you're ready to broadcast. You can even become a partner and make money off of your videos. As a side note, it is estimated that in 2007, YouTube consumed as much bandwidth as the entire Internet in 2000, and that around ten hours of video are uploaded every minute.
http://www.youtube.comQOOP
If flickr uses it, it must be good. While Lulu mainly handles larger print publishing, QOOP can handle everything else. QOOP allows you to turn your photos into printed products like photobooks, prints, posters, postcards, shirts, calendars and more. They have a great API that they won't let anyone use. Yes, you read that correctly.
http://www.qoop.com