One of the best things about Google (and part of the reason for its success) is the way its engineers think of themselves as much more than a simple, consumer-oriented search box. Their search box is actually a sophisticated command line that lets you do very complicated stuff once you know what you're doing. I bought a copy of Google Hacks even though the full text is available online through Safari (which by the way, is free if you go through home pages like the Sunnyvale Library) because I wanted to learn more about these searches.
Here's my favorite: it returns music files of certain types with the word Nirvana in the link from certain pages all over the web:
-inurl:(htm|html|php) intitle:"index of" +"last modified" +"parent directory" +description +size +(wma|mp3) "Nirvana"
[see LifeHacker for more details]