Mac: Like the Windows beta, there's no real feature in Kindle for Mac, beyond Kindle e-book syncing, that will rock your socks off. For doing some reading away from your Kindle, though, it works just fine.
The advantages to a desktop client like Kindle for Mac over a Kindle device or iPhone are slim, but notable — colour pages, a wider screen, and presumably better and faster page and scroll controls. Then again, Kindle for Mac lacks the ability to add notes at the moment, but can add bookmarks and show both notes and bookmarks.
If you've got a decent Kindle collection and a Mac, it's a free, no-brainer download for Mac OS X 10.5 and above.
Kindle for Mac [Amazon.com]