Lyricsnapper is a free tool that retrieves lyrics from the internet. It can be used in two ways. By itself, you can enter the artist name, and song title and the lyrics for that song will appear.
However, the most useful way to use this tool is as an add-on to iTunes. When connected to iTunes, it will automatically download lyrics as songs are playing.
This iTunes add-on can update the "Info" of your iTunes track to include the lyrics. You can modify the lyrics that are returned as a search, and save those to your iTunes library.