Check out Andy's new web application: upcoming.org. It lets you track events (concerts, conferences, etc.) that you're going to—and browse events that others have said they're going to. He calls it a collaborative event calendar. All of the events are organized by location and user, and you can keep a list of your friends on the system (and see what they're up to). He has everything available as XML, so you can add your own upcoming events to your site with some fetching and parsing.