Rich clients and web applications
That said, I love it when web portals open up their systems with public APIs. Flickr's API is especially nice since they support REST, XML-RPC, and SOAP. For Mac OS X 10.4 users, there are two great Flickr rich ("fat") clients that I think are very cool: Flickr Export for iPhoto and tickr for Flickr which allows you to search on Flickr tags and see a scrolling list of matched thumbnail pictures that scroll up the right side of your desktop.
The important thing here is that Flickr did not have to write these fat clients - they just had to make a public web services API available.