Game Engines

Panda3d

Panda3d is a Python based Game Engine that is Open Source and Free. It is well documented and you can find most functions or properties in the docs.
Panda3d Website

Godot

Godot Game Engine offers common 3d functions and properties in a nice GUI.
Godot Website

Graphics

Inkscape

Inkscape is software for making vector drawings and other technical drawings.
Inkscape Website

GIMP

GIMP is an Open Source Raster Graphics Editing and Creation software.
GIMP Website

Modeling and Animation

Structure Synth

Structure Synth lets you code objects in an additive synthesis method, starting with basic shapes and functions that come together into more complex objects.
Structure Synth Website

Blender

Blender is software for 3d Modeling, Texturing, Rigging, Animation, Shaders, Compositing, Video Editting, and more!
Blender Website

Music

FL Studio (Non-Free)

FL Studio is software for Writing, Composing, Sequencing, and Mixing music tracks.
FL Studio Website

OpenMPT

OpenMPT is music tracker software, you can add samples, Compose, and Sequence patterns of Tracker music.
OpenMPT Website

AlterEgo

Alter-Ego is a program that allows you to take voice samples and write lyrics to be sung by that voice; Free Vocal Synthesizer.
AlterEgo Website