I’m not a big fan of the Kindle, but some people do like it. Amazon already has an app for the iPhone, but now the company released an app for Mac OS X too. The app requires Mac OS X 10.5 or higher.
The app allows you to download Kindle books you already own, and read them at leisure on your Mac; synchronization of your progress through the book is automatic, and will keep track with your Kindle or iPhone reading.
9 to 5 Mac has some more information, as well as the press release from the company.
Amazon releases Kindle Application for Mac [9 to 5 Mac]