If you are looking for older outliners, check out the Antique outliners section.

• Windows
  • ActionOutline
  • AnyNotes
  • ADM
  • Buzz
  • BrainStorm
  • DaRT Notes
  • Frontier
  • Genius
  • Groove Outliner
  • InfoSelect
  • InfoTree
  • Inspiration
  • Keynote
  • JOE
  • Jot+
  • Juristudents
  • Leo
  • MyInfo
  • My Life Organized
  • Natara Bonsai
  • Nota Bene
  • NoteLens
  • Notemap
  • OPML Editor
  • Outthink
  • PDesk
  • Quick Outliner
  • Radio UserLand
  • ShadowPlan
  • Skwyrul
  • StoreLaw Outliner
  • Tkoutline
  • TreePad
  • TVO
  • VimOutliner
  • WhizNotes
  • WikidPad
  • Zoot

• Mac
  • Deep Notes
  • DevonTHINK
  • Hog Bay Notebook
  • Inspiration
  • JOE
  • NoteBook
  • NoteTaker
  • OmniOutliner
  • TAO formerly FO
  • Tinderbox
  • TVO
  • VimOutliner

• Linux
  • Brainstem
  • hnb
  • TVO
  • VimOutliner
  • Woody

• Palm
  • Bonsai
  • BrainForest
  • Progect
  • ShadowPlan
  • ThoughtManager

• Web
  • In-browser XOXO Outliner Experiment
  • Sproutliner

• In development
  • Focus
  • GnomeOutliner
  • Iron Lute
  • OPML Creator

• Online OPML viewers
  • Olive
  • OPML Browser by voidstar
  • OPML Browser by yabfog
  • OPML Surfer
  • Scott Isaacs' "Ultimate Outliner"