I'm evaluating via the trial version and I like what you've done. Here's a few suggestions for improvement.
1.Hot keys. You've got all the functionality in place but it's very mouse-centric.
Stuff like press 'DEL' to wipe a note rather than access Delete via the mouse.
A hotkey for inserting a new text note
A hotkey for renaming a note etc.
2. Autonaming of new notes.
Rather than having to manually name a new note it would be great if the program could use the first line of text in the note as the default name. Eg. So you paste a new monster into a note and the monster name at the top automatically becomes the note name in the sidebar.
In many ways The Keep is similiar to a PIM called "INFOSELECT" . Although this is a fairly mature product some of it's basic functionality could be handy.
3. GM Window for text notes
This is a great feature but having it restricted to images is limiting. It would be handy if it could be a universal button for any note type.
You've got a great idea and are well on the way to a great product here. As mentioned above other products provide a lot of similiar functions but the ability to integrate your other product line within The Keep is a winning edge.