Blender is a professional free and open-source 3D computer graphics software product used for creating animated films, visual effects, and art. Blender's features include 3D modeling, texturing, rigging, fluid and smoke simulation, particle simulation, soft body simulation, sculpting, animating, match moving, camera tracking, rendering, video editing and compositing.
SketchUp is a 3D modeling computer program for a wide range of drawing applications such as architectural, interior design, landscape architecture, civil and mechanical engineering, film and video game design. It is available as a freeware version (SketchUp Make) and a paid version with additional functionality (SketchUp Pro).
GIMP (GNU Image Manipulation Program) is a free and open-source raster graphics editor used for image retouching and editing, free-form drawing, converting between different image formats, and more specialized tasks.
Inkscape is a free and open-source vector graphics editor. It can be used to create or edit vector graphics such as illustrations, diagrams, line arts, charts, logos and complex paintings. Inkscape's primary vector graphics format is Scalable Vector Graphics (SVG), however many other formats can be imported and exported.
Krita is a raster graphics editor. It is designed to be a digital painting application. It has been influenced to some extent by software like Corel Painter and SAI. Krita packs with some key features different from most of its kind, including the ability to work with both bitmap and vector illustration. The developers also strive to simplify user interaction and focus on features more relevant to painting and drawing.