A good spectrum analyzer is a tool every beat maker needs when mastering. By using a spectrum analyzer VST you can see problem frequencies and target your final mix to be well balanced, thus sounding good in a variety of venues (car, club, home, boom box, etc)
Seven Phases have released a great and configurable freeware spectrum analyzer VST plug-in.
Click on the link above to check out the specs of this fine little freebie.
Click here to download the Seven Phases Spectrum Analyzer