Mac OS X only: Lightweight outlining application Tree builds outlines quickly and easily, then displays your outline in a novel left-to-right flow between child and parent elements. There's not all that much to say about Tree beyond that. It's lightweight and fast, is full of keyboard shortcuts for quick outline creation, and it can export any outline to OPML, meaning any outlines you make in Tree should be simple enough to import to most other outlining apps. Tree is a free download, costs a hefty $US40 if you want to upgrade to the unlimited version. (Incidentally, as the Download Squad post points out, that's the same price as the much more robust OmniOutliner, so you may want to give that a look if you're considering paying for the full version.) Got a preferred outliner you're already using? Let's hear about it in the comments.