Columns are great for many uses. But there is another two-dimensional display that is popular and useful: nested diagrams of different sorts. There are several different philosophies connected to such displays, some reaching religious fervor. What you will prefer is going to be a highly personal decision.
One category of these are diagrams that place headers at nodes, showing the child relationship as a line or arrow. Inspiration is the most flexible of these and is shown in the screenshot. It allows all sorts of graphical control over layout and the graphics assigned to headers. Outlines can be edited and browsed in the diagram view. These are tree diagrams and often look like starbursts or trees or (if they go from top to bottom) like organizational charts. MyMIND does this in a rudimentary way. Some newsreaders show news item threads graphically in this way too, and you could consider them graphical outliners.
One style of tree diagram is the “mind map,” a rigid set of graphic conventions that is supposed to expedite brainstorming and has many adherents. If that’s what you need, NovaMind is your outliner.
Another approach to displaying trees is to consider them as containers. A parent might be a rectangle that contains another rectangle that is a child. This has a certain economy that then allows for the graphical display of links among elements. That’s what Tinderbox does, using tools developed over a decade. Pretty powerful.
OmniOutliner outlines can be imported into its sister product OmniGraffle, a charting application, but the transition is a bit lumpy and only one-way. Greater integration is planned.
Schedule has an amazing variety of views: chart, resource, and calendar views, all relating to its outline.
Some things that are not primarily outliners, or are at least not good outliners, will ofter offer an outline-like view of such graphical things. Programming editors will ofter offer an outline-like display of various programming entities of interest, even if you can not directly manipulate the outline itself. Graphics displays that allow things to be created via complex composition will often have outline views of the individual and composite components available; this is common in 3D editors. Most presentation software will give you an outline view of the presentation.
Almost anything that is ever stored as XML or a similarly hierarchial storage language can be view as an outline. If you view the source of this page, while HTML is technically a flat-text-based language, you can sort of see the page outlined for you, as the Wiki engine used to create this site outputs nicely-formatted HTML. It is common practice in Frontier, for instance, to directly edit HTML in an outline, which helps you keep track of the myriad nested components used in any real web page without getting lost. Outliners make great XML editors, and it’s the rare XML editor that hasn’t at least taken a few cues from an outliner, even if it hasn’t gone whole hog.