Lyrics by musiXmatch is easily one of my favorite apps so far on Windows Phone 7. Not only does it expand upon Zune in an useful and intuitive way, but it's free and just really cool.
In short, the program looks up lyrics to whatever song you want--either by search, going through your library or by showing you the top artists of today. It pulls down images and bio, just like Zune does as well. But the real big thing is how it integrates into the Zune hub, allowing you to seamlessly look up the lyrics to any song you are currently listening too. The program just "knows"what song is playing and it grabs the lyrics--very nice. Also, if the lyrics are wrong, there's a feedback mechanism to report the issue.
Overall, the program is highly accurate and pulls down all but the rarest of artists. If you enjoy reading the lyrics to some new music and want to do it effortlessly, then this app is for you. It's also completely free, so no reason to try it.
Grab Lyrics here (opens Zune Destop)