Every good work of software starts by scratching a developer's personal itch
Browsing through the firstname.lastname@example.org archive I found Mark Overmeer's OODoc application.
It tries to solve a common problem with documenting classes and inheritance hierarchies; classes inherit from each other, but the documentation local to each class isn't necessarily presented in the same manner. The interface of a subclass is fragmented without the parent class taken into account.
In general, method overriding and late binding can make OO code difficult to understand, so decent docs/class browsers are often vital aids when trying to form an understanding of OO systems.
From university, I remember the language Eiffel dealing with this by providing a flattened view of a class and it's ancestors, both the docs and the actual code. Very useful.
So with that in mind I wrote something similar to merge POD sections from inherited classes a couple of years ago. It never felt stable or mature enough though, so I never released it. But other people seem to experience the same problem with documenting class hierarchies, scratching the itch as it becomes evident.