It's a plugin-extendable document browser. You can use it to manage a library of documents and display them in the tabbed browser on the right. It comes with plugins supporting text files, Rich Text documents, and anything IE supports (so in this case it's HTML and PDF).
I've also wrapped the XP Picture and Fax viewer as another plugin, but am having a few problems with that so I'm not sure if I'll end up writing my own image viewer. Multipage TIFFs would be nice, though.
Plugins can expose their own toolbars - hence the text file has a dropdown button with a list of encodings, which has been very handy for those hundreds of DOS text files I have knocking around.
The library is stored in an XML document, and I'm intending on devising a format for easy distribution of documentation - probably something as simple as a Zip archive containing the relevant files and a small XML document that can be merged with the main library one.
Two nifty links, if you didn't know about them already - famfamfam's glorious "Silk Icons" set and the oh-so-useful SkyBound Visual styles (notice how my tabs on the left of the screenshots render correctly, even though they're upside down).