For example, instead of an online ad for amazon.com, why not allow people to use amazon to buy books from other sites such as acm.org. It would look like it was designed for acm.org all along with no obvious indication that this is coming from another site.
Taking it further, one might even restrict the selection of books to those that would interest acm.org visitors.
One problem though is that of handling accounts. Requiring people to register on acm.org to buy a book when they already have an amazon.com account is questionable -- though doing so would preserve the illusion that this is all acm.org.
Another problem is that seamless embedding requires an amazon.com UI that fits in with the rest of acm.org. Trying to automate this would present some interesting research problems.