Specialized

Demucs Music Source Separation - Demucs is a state-of-the-art music source separation model, currently capable of separating drums, bass, and vocals from the rest of the accompaniment. It's under MIT license.
TouchDesigner - a visual programming language and development platform, primarily used for creating real-time interactive multimedia content and applications

Utility Tools

Loudness Penalty Anylyzer
Composers Desktop Project - is a comprehensive sound design suite of over 450 processes, covering almost every aspect of sound transformation, including many that may be unfamiliar or are presented from an original or compositional viewpoint. The system is not real-time, but is non-destructive: most processes take one or more input files and produce an output file.
SoundThread - a cross-platform user interface for The Composers Desktop Project (CDP) suite of sound manipulation tools. It allows for modular style routing of various CDP processes to quickly build up complex Threads that allow for extensive sound manipulation. The goal of SoundThread is to make CDP as user friendly as possible and it is particularly well suited to those new to experimental sound processing.
AATranslator2 - A comprehensive session file conversion program for the audio and video industries. It allows you to convert sessions between one DAW and another, and between DAWs and NLEs. This flexibility is liberating since you can now work in your favourite DAW/NLE without worrying about compatibility and you can also use the program which is most suitable for the task. With AATranslator you can accept or send session files in virtually any format. All this is good for creativity and helps get better and faster results.

Plugindoctor - A cross-platform and cross-bitformat plugin analyzer for VST and AU plugins. Whether you are a plugin developer yourself who wants to thoroughly test the audio quality of your latest and greatest product, or a power user who wants to understand in more detail what exactly a specific plugin is doing to your precious audio material, Plugindoctor will help you find out!
plugins4free
Signalizer - is a all-in-one signal visualizing package with a bunch of unique focus-points; real-time audio visualization with optimized 3D GPU graphics, everything being scalable and zoomable gridlessly as well as being arbitrarily precise in both settings and display. Combined with a rich feature set, Signalizer is suited both for electrical/audio engineers fullscreen-inspecting signals, or for general small windows giving an overview of your audio as you create it.

Music Theory Site
hook theory
Plugdata - A visual programming environment for audio experimentation, prototyping and education