Earlier, Amazon released the Kindle for Mac software on the Mac App Store.
While the Kindle for Mac has been available for a while, always as a free download. The significance of it showing up on the Mac App Store is that it signals that major players are taking the Mac App Store seriously. What remains to be seen is if other major developers like Microsoft and Adobe will also add their apps to the Mac App Store. If they do, they’ll be giving Apple a 30% cut of their profits. That is, unless Apple cuts a better deal with them to get their major apps in the store.