Playscii - Full-featured ASCII art and graphic editor and game creation tool
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
draw.io - Online diagramming application
LaTeXDraw - LaTeXDraw is a graphical drawing editor for LaTeX.
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
Ipe - Ipe is a drawing editor for creating figures in PDF or (encapsulated) Postscript format.