If you're wondering about the people who subscribe to your Twitter feed, there's a newish service out called Twittersheep that will tell you about them. It scours your list of followers and pulls together a tag cloud with various keywords to describe them.
If you remember Tweetclouds, the idea is similar, except instead of going through your posting history, it simply grabs its words from the profiles of those who are following you. This isn't an exact science, but it made me notice that a good majority of the people following me are work-related, be it PR people or staffers from companies we profile.
The site requires your Twitter log-in to do its business, so you won't be able to use the service with a friend's name. It does however have a search tool that will let you search for words that are getting buzz. In future versions, I'd love to see some additional metrics like how active your followers are with their own tweets, and things like age, location and gender.