newTunes seems like a pretty cool dashboard widget. It will let you know when new music is in the iTunes Music Store for any of the artists in your iTunes library.
Unfortunately, it doesn't work well for me. I installed it on Friday, and on Monday it still doesn't display anything. The problem is that the widget launches an AppleScript that, I believe, is building the list of the artists and the local tracks for those artists. On my library that has 7800 tracks, this takes a long time.
I think that it would be better for this widget to parse the iTunes Library xml file itself. (I believe that Apple has an API to do this, this is what iPhoto uses to show the list of tracks for slideshows.) This would be a lot faster, and would allow the widget to show status while the list is being built.