Breath Flutes are musical instruments that are very easy to play - they have no finger holes and you play the instrument with your breath. They are in a class of flutes called "overtone flutes" which change pitch based on how much air pressure you provide. They are useful in community music sessions as well as in music therapy, therapeutic music, and a wide range of personal breath practices such as yoga and meditation.
The Breath Flute Project is a GPLv3 Open-Source project that provides a highly parameterized OpenSCAD model as well as 100+ pre-rendered STL files of various options and configurations. The release package also has extensive documentation (over 300 pages). Here is a direct link to the Developer's Guide (63 pages / 1MB).
The various STL files let users print the 5" tall headjoints as well as custom sanding tools to facilitate post-processing. You add 3-4 feet of 1.25" PVC tube to a printed headjoint to make a complete instrument.
This ThingIverse posting has a single STL file that demonstrates a full print of a headjoint. However, for a number of reasons, most headjoints are fabricated in three separate prints and joined with set screws.
Everything related to the Breath Flute, including the full distribution package, is available at www.BreathFlute.com.