Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
TiddlyMap - An interactive concept- and mind-mapping plugin for the personal note-taking software TiddlyWiki based on the Vis.js library. It's free and open-source!
LaTeXDraw - LaTeXDraw is a graphical drawing editor for LaTeX.
Wisdomap - Wisdomap Medicine is a study aid that helps medical students create differential diagnoses and suitable management plans from a presenting complaint and structured revision notes.
Ipe - Ipe is a drawing editor for creating figures in PDF or (encapsulated) Postscript format.
IOctopus - IOctopus is an online mind map service for a dive deep into the mind for the best ideas.