ScreenTunes is a free search tool that helps users find where a song has been played in any piece of professionally-produced video content. You just type in the name of the song and it tells you all the films and TV shows in which it's appeared. You can then listen to the track right from the results (using Grooveshark), or purchase the track on iTunes or Amazon's online stores.
Along with acting as a search tool for individual songs, ScreenTunes can be used to look up a track by its lyrics. This also works on entire movie titles, so you can type the name to get a quick track list.
I found the service to be a bit hit or miss, but when it works it's quite neat. Many test searches I did came up with zero results, often times because I had either left a word off the title of the song, or it simply wasn't in the database. Clearly you get better results if it's a mainstream track, but it's surprising to see how many places any one song can end up.