I'm normally not a huge fan of iTunes plugins– I was scarred on the whole idea of media player plugins back in my Windows days, but iConcertCal might just get me to re-evaluate that opinion. The free plugin scans the artists in your music library and presents you with a calendar of upcoming concerts in your city by those performers.
Unfortunately I haven't been able to get iConcertCal to work perfectly on my machine– the calendar only fills the bottom-left 2/3 of my iTunes window, the calendar over-flows the bounds of the visualizer area a bit, and there is something wrong with the mouse rollovers that causes me to have to mouse about an inch above the item I want to see information about. Your results may vary.
Technical note: Now I'm no programmer or expert on the intricacies of iTunes plugins, but it seems to me that having a plugin that essentially uses the visualizer as a browser is a bit of a hack.
[via Josh Spear]