these are my essentials:

• blender -> 3d graphics software, and with the blenderbim addon it turns into an architecture/engineering beast,
• freecad -> parametric 3d drawing program, amazing for 3d printing, also has a bim addon,
• qcad -> cad drawing software,
• qgis -> topology software,
• kicad -> circuit design software,
• krita -> it used to be just a painting program, lately it's become a great graphics software,
• gimp -> it's always contested but still my favorite image editor,
• ardour -> my favorite daw,
• lyx -> great latex editor,
• at last but not least: vim -> the greatest editor of all time