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.
Dynalist - Dynalist is a web app that lets you break down and organize your thoughts in the format of lists.
Outspline - Extensible outliner and personal time organizer to manage todo lists, schedule tasks, remind events.
OutlineEdit - A beautifully crafted outliner for your Mac: