eBay has launched eBay ToGo - Flash-based widgets you can embed anywhere that allows Flash content. Here is mine:
I consulted on this project, which means that I was asked to some of the meetings and got to spout of my opinions without having to do any of the actual work. I think the best part about these widgets is that they really push the boundaries of offering up useful functionality without forcing you back to the website. Try the search box - it lets you search eBay from within the widget itself.
eBay ToGo (not eBay Togo - thats an island in the South Pacific) is of course built on eBay Web Services - specifically our REST API. As I point out in the Techcrunch article announcing the launch, we have some great new services that we are working on that we will be showing off at our Developers Conference in June.