Sing That iTune! is one of the best lyric fetching tools you will find on Mac. It's clean, fetches the lyrics automatically for you and interacts well with iTunes.
Having it as widget means it is immediately available. You will have to be running iTunes to make it work though. You can either search for the lyrics of a song at the back of the widget, by typing the name of the song or of the artist, or just start any song in iTunes. Sing That iTune! can be a little erratic, and you might find it freezes up or won't display lyrics. However this isn't as frequent as you'd think.
I like how Sing That iTune! has links to YouTube and Google so you can look for videos and extra information on a song or artist. You'll find you can also search for album artwork, which is nice to have since you need an Apple account to do this in iTunes.
Sing That iTune! also lets you save the lyrics of a success with the music file. If you want them separate you can select a folder to place them into.
Sing That iTune! is a clean little widget that will search for and display lyrics to your iTunes tracks.
