Here's a simple use for Quicksilver. Create a new trigger with the first pane set to your music library. Set the second pane to "Search Contents" and now you can load any song while working in any application. Shouldn't iTunes come with this feature built in?
Another way to do this is with the iTunes plugin for Quicksilver. Invoke QS and start typing "iTunes", then right arrow in to see a list available options like browsing artists, tracks, genre etc. This is a pretty nice interface as well.