Kabikboo is an open-source application written in Python. It organizes information into a hierchal tree, according to the user's specifications. In Kabikaboo, everything is a 'node'. Each node can contain child nodes, and each node can either be an information node--containing notes, for example--or a 'view' node--organizing and displaying the information in all the nodes below the view node. So, for example, you could define a node of "Characters"--make it a view node. Beneath that would be nodes for each characters, and possible nodes beneath *those* nodes for each character's backstory, motivations, and so forth. The Characters node would display and sort all the information for you!
It's a little hard to explain without images. Here's a screenshot of Kabikaboo 1.7 under Ubuntu Linux. Of course, the nodes are not pre-defined--that's the beauty! The writer has the ability to define nodes to his or her liking.
Kabikaboo was developed on Ubuntu Linux, but I have put together a Windows port for version 1.6, which can be found at this location. The installer is named "kabikaboo-1.6-setup.exe".
Ubuntu users can add the PPA to their apt sources list; the PPA repository is listed on the Launchpad site.
Links:
Kabikaboo website
Windows downloads