I browsed around Google Labs the other day and found Google Sets which seemed quaint but not all that useful. Then I read about how other people suggested using the service and I found a few cool applications.
It can be used to recommend new musicians. You put in your top three favorite artists and see who else it includes in the set.
Find out all the cities in a state by entering a few that you know.
Other categories I thought of include the fifty states, cars, restaurants, planets in the solar system, presidents of the United States, entertainers and dozens more. In my tests there were a few odd entries that didn’t fit so it’s not definitive, but if used in conjunction with other research you can filter out the anomalies.
In short, if you know a few items from a set and want to know the rest of them, it can be a handy little tool.