Workflowy - A better way to organize your mind.
EditiX - EditiX is a quality Open Source XML Editor and XSLT Editor for Windows, Mac OS X and Linux. Test for Free with our 30-day Trial !
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.
Firstobject XML Editor - Firstobject's free XML editor for Windows is called foxe.
TreePad - A free and lightweight outliner, only 465KB in size.