Visio was the only Office application to be entirely developed outside of Microsoft before being purchased and absorbed into the suite. It is, to this day, the best one in the bunch, and the only one I really miss in Mac OS X.
Resources:
- Crime Scenes With Shapes - fun for all the family.
- Visio Cafe
- OmniGraffle - the closest Mac OS X analog
- VisioAutomation - a .NET library for automating.