This is one of the coolest things I've seen in a long time.  Try my flickr name (marklio).  It lets you drill through my contacts and see their information.  Keeping clicking and you'll see all kinds of cool things.  It really helped me see Flickr as a web service rather than just a photo sharing site.  It gave me alot of ideas.  Things like using tag matching to automatically creating blog entry backgrounds that are relevant to the content.  I'm going to be playing with that.

Their API is really nice and well-documented.  Some people made a .NET wrapper for the API, but it's not up-to-date and I wasn't really impressed with the design.  I'll probably roll my own and just implement the things I need as I go.