I seem to sign up for one of these services every year or two, and then one day, the alerts stop coming, presumably because the service goes out of business.
BandsInTown helps you stay abreast of what bands you like are in town. You drop in your Last.FM or Pandora ID, so the service knows what bands you like. Then, through a combination of online tools and email alerts, you will not miss any of your bands when they come through town.
Let's hope these guys can stay in business.