It would be great (for me at least) to be able to do several common activities on my Kindle. The main motivator is that I find it hard to sleep if I've been looking at screens late at night. For example, I'd love to be able to use some kind of messaging client.
I was never able to get Kindle Active Content (the closest thing there was to an app store) as I'm not US-based and that seems to be disappearing anyway.
Which means the only available option is using web-apps. Of course, the obvious problem is that they're totally unoptimised for e-ink screens where scrolling, colour and movement are all either poorly implemented or not available at all.
Are there any web-apps designed to work well with e-readers?
With regards to eink web-apps, don't forget you'll also have to fight the keyboard if you'll even get one. So it's hard to design to that medium. On that note, there's the onyx eink phones/tablets that have a version of the play store, but their only unique apps are rss reader and google calender. But maybe sniffing around onyx and their users will let you discover usable apps.