There's nothing better than having the lyrics to your favorite songs with you on iTunes. Unfortunately, iTunes doesn't include a lyrics retrieval tool but you can get round it by adding this program.
iTunes Lyrics importer or "iLyrics" automatically searches the net when you click Get Lyrics and downloads the song lyrics based on their titles. However, you should note that iLyrics is a discontinued project due to copyright issues according to the developer. This means that it definitely will not work with iTunes 9 although it may still work with iTunes 7.0 and 8.0 for which it was designed.
iLyrics has a huge databank to choose from thanks to the Lyricswiki project which allows users to upload the lyrics of their favorite songs. Because of this, you're more likely to find the lyrics of obscure songs rather than other lyrics sites which concentrate more on mainstream offerings. To use iLyrics, you need to go into Tools in iTunes and select Lyrics Viewer.
Normally, iLyrics will give you a direct hit and provide you with exactly the song you were looking for. However, occasionally it will confuse the song with another one but you only have two modes "Update automatically" or "Manual Mode". If you select the former and it finds the wrong lyrics, note that you'll lose the old lyrics and they will be replaced by lyrics that may be wrong. If your initial experiments prove to be a bit dodgy, then I'd recommend using the latter mode which gives you control over whether to definitely replace the lyrics or not.
iLyrics is an excellent addition up to iTunes 8.0 which has a huge resource of lyrics to draw from although take note that the project is officially discontinued.