I’ve been using OmniGraffle for about a year now. It’s a really good tool that does what Visio did best in its early years: Diagramming, plain and simple. In the new 2.0 version there are several really nice improvements, but my favorite by far is the ability to drag a Cocoa framework (say, /System/Library/Frameworks/Foundation.framework) onto the OmniGraffle application icon and have it generate a nice class diagram for you.
It’s way cool and has helped me quickly get a sense for several of the newer frameworks and how the pieces go together. Along with Cocoa Browser, it’s become one of my tools for working in Cocoa and Objective-C.