Chris Bryant (one of the organizers of the upcoming Office Dev Con) pointed this out to me the other day, and I wanted others to know about it. If you're looking at OBA development, I'd recommend checking it out. It's a great way to get a look at the object model in a semi-graphical way. From their web site:

The Microsoft Office Interactive Developer Map is a Windows Presentation Foundation (WPF) application that helps developers visualize the different programs, servers, services, and tools that will help them build solutions. It allows them to drill down to each product and technology and learn about new features, objects, Web services, namespaces, and schemas required to extend Microsoft Office and build custom Office Business Applications (OBAs).