jDFT-2 is the free RTA (Real Time Analyzer) software for Windows.
Use it to analyze the frequency spectrum of audio in real time. Just connect a source to analyze and have fun! Best of all - it's free and easy to use.
jDFT makes use of DirectSound which is a part of DirectX, this makes it possible to analyze any audio source that's connected to your sound board. As long as your sound board is DirectX compatible.