There are a variety of interesting, publicly available, web services available on the Internet that give insight into the state of the practice for this technology.
One of the more interesting ones is at Amazon.com ( http://www.amazon.com ) which allows you to programmatically access many of Amazon's services.
Google also publishes and extensive, public, set of web services - see http://www.google.com/apis/
Microsoft's TerraNet provides web-services for go information - see http://terraservice.net/webservices.aspx
Some of these require registration, have non-commercial use restrictions, and/or limit you to a certain number of transactions per day.
For a list of publicly available web-services, check out http://www.xmethods.net/