iConcertCal is a free iTunes plug-in that monitors your music library and generates a personalized calendar of upcoming concerts in your city.
When iConcertCal first starts up it will try to automatically figure out what city and state you are in by checking your ip adress. But you can easily change your current city and state to search for shows in another town near you, or in a town where you plan to visit.
Currently iConcertCal only searches for concerts within the United States, but the next version (due out in a few days) will also support international searches.
iConcertCal is available for both Windows and Mac OS X.