when I've read about the new Amazon Kindle Developer Kit, the first idea that came into my mind was Bookcrossing, or better, Ebookcrossing.
Barnes & Noble's Nook reader already gives the chance to lend ebooks to friends, but from what I've heard the interface is very slow and it can't be compared with the Kindle. The Kindle on the other hand, doesn't let you give your books to other people, but if an E-BookCrossing app was available on the Kindle Store, I think it could be a game-changer.
It could be like BookCrossing but on the Kindle, and powered with apps that could let you leave ebooks on park benches, bars, airports based on GPS data. I can imagine that many games, like book treasure hunts, could be organized, and communities of book lovers and Kindlers could meet.
I wrote a blog post about it at http://www.bealoud.com/---/kindle-development-kit
What do you think about it? Do you own an ebook reader?
Feel free to comment on the blog post or please add your ideas here! :)