Just now Amazon quietly launched the Kindle for Mac.
The new applicaiton is a very basic that fucntions exactly what you expects a Kindle application on the Mac does. It downloads your purchased Kindle books from Amazon, so you can read them on your Mac. The application also use Whispersync to synchronize the spot in the book where you left off when you last read the book. The functionality is not that much different than the Kindle for iPhone app. The only major addition is the Mac version's ability to resize the screen displaying the book.
Unfortunately, there is a bug in the application, where if you were resizing the window (by dragging or clicking on the Zoom Out button) while the book is loading, the application often messes up the line widths of the book.
Un-intuitively you have to use the Font Resizing icon to bring up the Font/Line Resizing window to adjust the line width, so that more of each line fills the page.
Hitting the Reset to Default button in the window or dragging the slider all the way to the right expands the lines in the book to occupy the full width of the screen.