Workflowy - A better way to organize your mind.
Orgzly - Outliner for notes, tasks and to-dos
MemPad - MemPad is a plain text outliner and note taking program with a structured index.
UV Outliner - UV Outliner is a small and powerful single-pane outliner.
HackNotice - Find out when hackers steal your data or leak your identity
OutlineEdit - A beautifully crafted outliner for your Mac: