This is a strange name for a more or less familiar concept: a personal wiki. It "is an open source Web notebook application that provides you with a platform to build your knowledge personally or collaboratively." You are supposed to be able to "create highly structured content by connecting knowledge fragments to each other to build a network structure, which is more flexible and expressive than a tree structure. Fragments can also be classified using hierarchical tags."
There is also a blog where the author explains why he developed this application: "Before I created Piggydb, I had been using a Wiki for storing and organizing my thoughts, ideas, article excerpts, and anything else I wanted to write down. Although a wiki provides an extremely simple and flexible way to organize your knowledge in a network structure, I came to feel that it was not well suited for what I wanted to do. As I used it more extensively, I found that the data structure of a wiki was not flexible enough when I want to reuse some part of a page in different context."
"Another drawback of a wiki, for me, is that it encourages you to organize your knowledge in a top-down manner, that is, you have to select a main theme as a starting point. But I wanted to write down anything I thought could be useful and organize afterward, as needed."
Let's take the second point first. It is simply false that a wiki encourages organization in a top-down manner. This has more to do with one's own mind set than it has to do with wiki. I remember that when I first started using one, I also thought I had to follow the top-down approach. It was like using training wheels. It took me a while to let go of them, but a wiki actually "encourages" a network approach in which every topic is as important as any other—at least at the beginning.
The first point is more valid, though an application like ConnectedText allows "transclusion" and linking to a specific part of a topic as well. So you can "reuse some part of a page in different context."
That being said, Piggydb, despite its name, is a serious application. Some people will probably find it useful.