Dump Objective-C runtime information, peek into frameworks

Steve Nygard is the author of class-dump:

This is a command-line utility for examining the Objective-C runtime information stored in Mach-O files.

Why would you want to do that?

It’s a great tool for the curious. You can look at the design of closed source applications, frameworks, and bundles. Watch the interfaces evolve between releases. Experiment with private frameworks, or see what private goodies are hiding in the AppKit. Learn about the plugin API lurking in Mail.app.