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?