Get Lyrical will attempt to get the lyrics of the currently playing or selected tracks from lyrics.astraweb.com and store them in the track's lyrics tag.
(commercial) iStar allows you to do karaoke on your Mac properly, grab some easy to find files off the net, and use them in iStar. Learn the lyrics and sing along with your favorite songs.
(commercial) iStar Composer creates synchronized lyrics on the Mac to export to the iPod and iTunes. iStar Composer finds lyrics, but more importantly allows you to sync them to a song and then allows you to export the synchronized lyrics into a new music file (AAC) that will play/display in iTunes and on the iPod.
pearLyrics lets you lok up song lyrics and chords as you play a song in iTunes.