Spectrogram Pro does a real-time spectral analysis of the sounds around you. You can use it for discovering hidden images in music, analyzing birdsongs or capturing that annoying high frequency whistling of your electronic device.

You can choose between four FFT-resolutions, logarithmic or linear fequency and amplitude scale and four color-schemes and adjust the sensitivity. Also you can save the spectrogram to the photo library. This app is optimized for iPhone 4's retina-display, but looks great on older displays, too.