We talk to clients occasionally about pulling in content and data from various, popular services across the web (e.g., Flickr, YouTube, Twitter). Most of the time it stays in the conceptual realm and never makes it to the finished product. Perhaps it never makes the transition because we don't use real-world examples to illustrate our ideas.
Ser-vice is "a lifestyle brand focusing on clean aesthetics and modern lines." Their shirts and products sell directly to folks in the creative industry and to people who use the aforementioned services. Case in point, their home page pulls in photos through the Flickr API tagged with "serviceisgood." Not only does this involve their customers beyond the initial transaction, but it shows new visitors there are actual human beings buying their wares.
All said, this isn't anything that new and I'm sure we could scrounge up dozens of examples of this type. However, it's a great example, in my opinion. We should look to this as a reminder that we've barely tapped into the potential of all the free services available on the web.