When a webmaster inadvertently leaves the permissions on directories open, Google indexes them like regular pages. With some crafty code that utilizes some of Google’s advanced search features, you can find these music directories and download the mp3 files. Here’s the query:
-inurl:(htm|html|php) intitle:"index of" +"last modified" +"parent directory" +description +size +(wma|mp3) "artist|album|track|etc"
G2P basically does the same type of thing but with an easier interface—you don’t have edit this long cryptic string, just input the song for which you’re searching.
Warning: watch out for fake open directories like listen77.com, which will redirect you to a page where you can buy music. You can usually tell the fake ones because they have advertisements.