Tkoutline - Tkoutline is a single pane, cross-platform outline editor written in Tcl/Tk .
SuperNotecard - Introducing SuperNotecard. SuperNotecard is an online writing tool that features virtual notecards to help arrange facts or scenes, track details, organize paragraphs, and clarify your composition process.
Workflowy - A better way to organize your mind.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.
Foldout - The single-pane outliner that Windows users have been searching for for years, free!
Freeplane - Freeplane is a powerful and free software for building mind maps.