An outlining application by the Omni Group:
I have found it fairly useful in creating documents that lend themselves to an outline format. Elsewhere on this site, I discuss a process for using OmniOutliner to build presentations for Keynote, S5, and other formats.
As for version 3, I upgraded today. So far, I think it’s worth it. A few features I was wishing for were added, such as the ability to uncheck certain parts of the outline so that they are not printed. Useful for printing out abridged versions of an outline for specific purposes.
As for the Pro version, there are also a few features that I wish the regular version had, that are available in the Pro version. But I am not sure if that additional upgrade is worth it… We’ll see.