The CMU Midi Toolkit (CMT) is a collection of software for writing interactive MIDI software in C. It includes a number of handy utilities along with an application "shell" that provides timing, scheduling, and MIDI interfaces that are portable across DOS, Mac, SGI, and Amiga platforms.
The Python audio processing suite is a module that contains a variety of convenience functions to process audio signals. It can be used to plot spectral analyses of a song across time and to quickly ascertain encoding quality, but the instrumental goal of this suite is to robustly identify duplicated songs, regardless of which album they were released in, encoding quality, or start time.
ll-plugins is a small collection of LV2 plugins and a host that runs them. All plugins are installed in separate LV2 bundles (except the ones that are closely related, like the math-constant plugins or the mono and stereo versions of the peak meter). The GUIs, for the plugins that have GUIs, are installed in bundles of their own to make it easier for packagers to put them in separate binary packages to avoid Gtk dependencies for the plugins themselves. There are synths, event processors, simple audio and control manipulators and GUI-based plugins.
Waterfall is a visualization plugin for XMMS. It's yet another spectrum analyzer, but it presents the data better, which makes it more interesting to watch. Most analyzers show a changing 2D bar graph of frequency vs. intensity. This one shows a scrolling 4D plot of frequency, time, intensity, and a configurable fourth variable mapped to hue.
libmls is a library that can be used to measure the frequency response of a loudspeaker or a room using your computer and an ordinary soundcard. It can perform both MLS (Maximum Length Sequence) and Sinus sweep measurements. It supports ALSA, OSS, and the sound system used in Windows.