FreqTweak is a tool for FFT-based realtime audio spectral manipulation and display. It provides several algorithms for processing audio data in the frequency domain and a highly interactive GUI to manipulate the associated filters for each. It also provides high-resolution spectral displays in the form of scrolling-raster spectrograms and energy vs frequency plots displaying both pre- and post-processed spectra.
It is an extremely addictive audio toy, but I hope it has value for serious audio work too (sound design, etc). The spectrum analysis is pretty useful in its own right.