Ta. Some (probably obvious) ideas:
* Better manga reader - the built in one seems rather buggy!
* Something like chrome-to-phone to get links onto the kindle more easily.
* I think the kindle has some sort of USB host capability. I'm not yet sure if the external USB connector is switchable, but if it is, attaching a mobile phone with camera would make an instant document scanner...
Myself, I'm now in the middle of writing a infocom text adventure kindlet for it; I think the kindle is ideally suited for that!