Instamac is a feature packed and easy to use Instagram client for Mac and one of the closest things you'll find to Instagram for Mac .
Instamac features classic single image scrolling view, grid views, multi-touch gesture support and easy image enlarging. You can also view people's profiles, follow people, and tag/tag search for images and friends. It's not perfect by any means though. It's not integrated with iPhoto or Aperture for example and it can be unstable at times but generally, it offers one of the best Instagram experiences possible so far on Mac.
As with Instagram for iPhone, you can view the most popular images in Instagram and Instamac also uploads the images you favorited to your account and view them all at once. In addition, there's a slideshow feature for photos with specific tags. Finally, there are notifications for new photos which is integrated into both Mountain Lion Notification Center and Growl and support for multiple accounts.
Please note that the early release of Instamac does not support Lion - only Mountain Lion - although the developers are working on a fix for this. It will also be updated shortly for Retina displays.
Overall, Instamac is probably the closest thing to Instagram for Mac although there are some big fixes and improvements needed.