Tkoutline - Tkoutline is a single pane, cross-platform outline editor written in Tcl/Tk .
Orgzly - Outliner for notes, tasks and to-dos
Workflowy - A better way to organize your mind.
MemPad - MemPad is a plain text outliner and note taking program with a structured index.
Foldout - The single-pane outliner that Windows users have been searching for for years, free!
Dynalist - Dynalist is a web app that lets you break down and organize your thoughts in the format of lists.