Some interesting services have emerged in the area of visualizing "affinity" networks as applied to different categories.
Live Plasma (previously Music Plasma) is a really cool service built on top of Amazon web services to support searching or exploration of music or movies. The user starts off by entering a band name to discover other bands that are similar (as determined by the Amazon collaborative filtering algorithms). As shown, similar bands are clustered and linked together in an interactive graphical display that makes for fun exploration. Clicking on the band shows their discography with links back to purchase the CD from Amazon. What could be really interesting is to link such a system together with a streaming service such as Napster so that the user can actually listen to the music.
Another exciting service is from introNetworks that supports discovering other peers with similar interests or affinities at conferences, corporations or other business networks. Users profile themselves by choosing attributes from a pre-defined list and interactively discover other people they would live to meet.
What is common in both the above services is the rich visualization layered on top of the similarity metrics. Its a matter of time before this is applied to other areas..John Battelle writes about about applying a similar approach for exploring relationships in the blogosphere.