Edgeio, http://edgeio.com, launched last week. Edgeio allows blog publishers to use their own sites as ecommerce storefronts, and uses tags and RSS to aggregate blog publishers' listings. On the buyer side, it provides a purpose-built interface for browsing these listings to find what you need. I think this is a good example of a "Web 2.5" application in some respects, in that it follows some of the principles of Structured Collaboration (see http://www.octavianworld.org/octavianworld/2005/12/what_can_web_15.html).
Edgeio was covered in Business week's blogs earlier in February:
Nik Cubrilovic's post in TechCrunch (a great resource) was also excellent, and the comments provide invaluable market feedback for the Edgeio team (rendering a lot of what I might have had to say superfluous).
Finally, Nick Carr's post on Rough Type, skeptical but partially right on:
What he misses, for my taste, is that these services are not intended to grow to the sky. They are experiments that ultimately make sense only in the context of a larger partner that builds or buys them.