Lyrics for Google Chrome is a popular Gogole Chrome add-on that will show you the lyrics to any song on YouTube, Last.fm, Grooveshark and Google Play.
An icon will appear on the address bar when a song is playing on any of these sites. You can choose to display the lyrics window in a new tab, window or integrated with the site.
You can even search or add lyrics manually if they aren't found by default.
Lyrics for Google Chrome is cross-platform and it works on Mac OS X, Windows and Linux.
Note: to install Lyrics for Google Chrome, drag and drop the extension file onto the Extensions window.