Several .COM leaders publish API that allow other developers to integrate their applications offered by these companies. Ebay has a developers program that allows third party developers to automate auction list posting.
Amazon has several programs. You can link to Amazon.com from your Web site and earn up to 10% in referral fees. Or one can use Web Services to query or sell items on Amazon.
Google also has an API that allows us to search Google and get results programmatically. However, there are too many limitations to make the API useful.