OmniOutliner is a popular, long-lived tool for building and organizing informational outlines--whether you're taking notes for a class, building out a complex project spec, or anywhere in between. OmniOutliner can help you create rich, multi-column, collapsible outlines in almost any style or format, with support for embedded notes, images, links, and more. Novices will have an easy time getting started with this flexible app's well-designed interface, but there's also plenty of room for power users to learn and exploit OmniOutliner's more heavy-duty features, including batch searching and support for AppleScript.
Given the abundance of competitive "mind-mapping" apps out there, some may find OmniOutliner's price tag a little steep--so definitely check out the generous educational discount if you qualify for it. The standard version provides plenty of muscle for most users, but hardcore outliners should try out OmniOutliner Pro for additional features like audio capture, dynamic HTML export, and support for third-party plug-ins.