I've been thinking a lot about how tags might be used to organize friends/contacts. Ideally I'd have a way to tag everyone I knew, perform basic set operations with the tags, and then synchronize the resulting lists with a variety of social websites. So I could be "friends" on a new site with, say, my 'friends among political junkies' and 'friends from college' but none of my 'friends from work' who were not also 'close friends'.