Org mode - Org: an Emacs Mode for Notes, Planning, and Authoring
Orgzly - Outliner for notes, tasks and to-dos
MemPad - MemPad is a plain text outliner and note taking program with a structured index.
Workflowy - A better way to organize your mind.
OmniOutliner - OmniOutliner is a writing app for collecting information, outlining ideas, adding structure to any sort of writing.
OutlineEdit - A beautifully crafted outliner for your Mac: