Impulse is a bit of eye candy for your desktop. It is a widget that displays a graphical spectrum analyzer on your GNOME desktop. It is written in C and Python, and uses GTK and Cairo graphics to generate the animation. The impulse library creates a pulse audio connection context that reads the output stream from pulseaudio in a thread natively, which can then be read from Python. You can instruct impulse to either output the raw stream or output the FFT of the raw stream.
RSound is a simplistic and lightweight framework for networked TCP/IP audio. It is designed to be very simple to implement, and to work well with music and video players. A librsound is supplied. It is not designed for low latency audio (games), and lets the sound backend take care of sound mixing. It supports POSIX-compliant systems (including cygwin). It includes audio output support for ALSA, OSS, libao, and PortAudio.
The dywapitchtrack library computes the pitch of an audio stream in realtime. The pitch is the main frequency of the waveform (the "note" being played or sung). It is expressed as a float in Hz. dywapitchtrack is based on a custom-tailored algorithm which is of very high quality, very accurate (precision < 0.05 semitones), with very low latency (< 23 ms) and very low error rates. It has been thoroughly tested on the human voice.