LMMS - Make music with a free, cross-platform tool
JavaScript - Lightweight, interpreted, object-oriented language with first-class functions
Anvil Studio - Free Windows multi-track MIDI/Audio recording & editing, VST Effects & Instruments, automation. Options for Sheet Music printing, music theory, ear training.
Allegro - Allegro 4 and Allegro 5 are cross-platform, libraries mainly aimed at video game and multimedia...
Cakewalk - The next generation of SONAR, the world's finest and most powerful DAW - now FREE.
Qt - Powerful, flexible and easy to use, Qt will help you not only meet your tight deadline, but also reduce the maintainable code by an astonishing percentage.