As of OS X 10.5, it’s possible—sometimes—to use Spotlight to search your iTunes music for lyrics. There are two fairly large caveats: first, Spotlight will only search MP3s, not AACs or any other file format, and second, you must have previously added the lyrics to the songs. This is useful, because you can’t search or use lyrics in playlists within iTunes.
Assuming your music meets both of those criteria, however, you can just enter tidbits of lyrics into Spotlight’s search box—either the one in the Finder or the drop-down dialog you get when you press Command-Space. Just like a regular Spotlight search, you can use quotes to search for phrases, and logical operators (AND, OR) to limit your search results.
You can use this trick to make Finder-based smart folders based on the contents of lyrics. Enter the words you’d like to use for the “playlist,” perhaps something like “love AND peace” or “red AND blue.” Set the first pop-up to Name and the second to “ends with”—then type “mp3” into the empty text box to limit the search to just your searchable music files. Save your search, and now you’ve got a “smart playlist” of sorts based on your lyrical search terms.
Unfortunately, the “MP3 only” limitation on this hint means that much of my music collection—specifically, anything I’ve bought from the iTunes Store—isn’t searchable, so I can’t say with absolute certainty that only two of my tracks mention “love” and “hate” and “war.” I can say, however, that only two of my MP3s contain all three of those words. (One from the Moody Blues, and one from the Josh Joplin Group; I’ll leave the identification of the tracks in question to you, the reader!) If nothing else, this hint could prove an interesting diversion as you dive into your music collection’s lyrics!