Piggydb is a flexible and scalable knowledge building platform that supports a heuristic or bottom-up approach to discover new concepts or ideas based on your input. You can begin with using it as a flexible outliner, diary or notebook, and as your database grows, Piggydb helps you to shape or elaborate your own knowledge. Piggydb is a Web application provided as a self-contained package that contains a Web server and database engine. With Piggydb, you can 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.