From the AutoGraf web site:
...a whole new metaphor for Cocoa with automatic, self-updating diagrams of your classes, and intelligent, visual browsing of code documentation.
Think of AutoGraf as the best way to understand your code. Using the AutoDoc code parser that is optimized for Objective-C, AutoGraf reads your existing Cocoa code and generates UML class diagrams rendered beautifully using Quartz and the AT&T Graphviz engine.
Open source, GPL license.
This is a useful tool, I find it easier to understand than with the xCode tools. The only gripe I have is that unless you subclass the cocoa classes they don't show up in the graphs.