Amazon has released the first beta version of Kindle for Mac.
Here is a features list:
– Get the best reading experience available on your Mac. No Kindle required.
– Access your Kindle books even if you don’t have your Kindle with you.
– Automatically synchronizes your last page read and annotations between devices with Whispersync.
– Create bookmarks and view the annotations you created on your Kindle.
– Search and browse more than 450,000 books, including 101 of 112 New York Times® Best Sellers. If you are a non-U.S. customer, book availability may vary.
– Find New York Times Best Sellers and new releases for $9.99, unless marked otherwise.
– Get free book samples – read the first chapter for free before you decide to buy.
– Books you purchase can also be read on a Kindle.
– Kindle newspapers, magazines, and blogs are not currently available for Kindle for Mac.
As for the requirements you need to have an Intel Mac running either Leopard (10.5.x) or Snow Leopard (10.6.x).