One of the most frequent questions I get from (new) users of Twitter is who I should I follow. A great question and I would usually forward them to a directory like Twellow (for categories and/or location) or Twitterholic (for location and number of followers). There are lots of Twitter apps for this but they are for me the easiest to use and describe and they can get immediate results.
But Twitter rolled out Twitter Lists and it has become my new best friend of the moment. Rather than go through the many, many followers of someone I like, respect, follow, or find interesting to see who they follow I can take a bit of a shortcut and see who of their followers they are listing. Sure they have missed some cool folks to add to a list but I will rely on them to point out their favorites.
Here are some examples of Twitter Lists (click photo to enlarge):