The Keep has become an indispensable part of my gaming life. It's a great one stop shop for the PDF rulebooks of all the games I'm currently running, and all the content I'm generating. It integrates with most of NBO's other offerings (Inspiration Pad, Character Sheet Designer, Screen Monkey, and Fractal Mapper). I use it to write scenarios, track combat, reference rules, create maps, keep various lists, and house images.
While it's an off-label use, I've installed the program in my Google Drive folder (using the thumbdrive installation method -- did I mention you can install it on a thumb drive?) which allows me to access the program and all the content I've put into it from any machine linked to my Google Drive account.
It's very useful to the pbp GM. Using character sheets generated with Character Sheet Designer and loaded into the Keep, I can update PC stats, then export the lot of them as images to a public Dropbox folder. In the game thread, I've linked to those images. So, it makes record keeping much easier.
Can't say enough good things about it.
[5 of 5 Stars!]