Tree: slick outlining app for OS X
Aside from the main selling point -- the horizontal Treeview -- Tree can also open and export outlines in OPML format, which means you won't have any trouble sharing with people who use most other outline apps, and you can also check out your favorite existing outlines in Treeview. Because Tree is meant to be lightweight, it doesn't have a lot of frills: custom fonts, colors and numbering are available, but that's about it.
All in all, it's a well-done app, but it also sports the same $40 pricetag as OmniOutliner, which I think is still the dominant Mac app in this category. Do you have another favorite outline app? Let me know in the comments.