"Sparks," by The Who, is a great instrumental rock song.
Neat link: If you're looking for new music, there's a website Pandora.com that can look up music with similar traits to the songs or artists that you enter into its search engine (saw it demonstrated on dltv podcast from a few weeks ago). What makes it more interesting than iTunes' recommended feature is, of course, that it's free, but it also breaks down all the characteristics of the song that might make a song "similar" to another, and it will usually describe them to you, traits like "mild rhythmic syncopation," "major key tonality," "subtle use of vocal harmony." I have to say from my last two searches on it ("Sparks," by The Who, or "Shadow Stabbing," by Cake), I've had some slightly disappointing hits; however, if you're like me, and you don't listen to the radio often but still ike finding/hearing new music, it's a good way to get at least a few good recommendations.
I think there was some sort of rumor of spyware at work on the site, but far as I know, they haven't been confirmed, and it doesn't seem to be bothering my computer at all.