Google Music Lyrics is a Greasemonkey script that adds a lyrics panel to Google Music. It only works in Firefox and there's a single lyrics provider: SongLyrics.com.
The lyrics are cached so that they're displayed instantly the next time you play the same song. You can also edit the lyrics and the changes are saved using HTML5 local storage. To hide the panel, click the title.
Chris Hendry, who developed the script, plans to add other lyrics providers, allow users to import and export lyrics and to make the script work in Google Chrome.