Users of Symmetry Software's Acta outliner - which was my favorite outliner back in 1993 (see "Inspiration 4.0: Outliners and Me," 14-Jun-93), although I never formally reviewed it - will be delighted to learn that its original developer, David Dunham, has rewritten it from the ground up as a Cocoa application and has released the result as Opal. Opal can open Acta documents, and its interface is reminiscent of Acta's, in its look and feel as well as its simplicity and intuitiveness. (Acta itself, meanwhile, continues to run in Classic, and is available for free.)
Unlike several other current outlining programs for Mac OS X, Opal doesn't have multiple columns, comments (notes), clones, or style sheets. What it has is outlining! You enter topics, you rearrange them, you navigate them, you collapse or expand them, and you view them in useful ways, including a "filtered view" that lets you see only those topics that contain specified text. A topic is styled text and can include multiple paragraphs, graphics, and clickable links to files on disk.
So, Opal will appeal particularly to those who want a straightforward outliner without extra bells and whistles. You might be a former Acta user. You might be a potential outlining beginner, who feels daunted by the complexity of OmniOutliner or TAO. Or you might be an experienced outline user who just wants to get work done with a minimum of fuss and barely any learning curve at all.
[Conflict of interest disclaimer: I had some paid, non-programming involvement with the development of Opal in its late stages.]
Opal requires Mac OS X 10.4 Tiger, and costs $32. A full-featured 30-day demo is available as a 1.9 MB download.