InstaDesk is one of the closest things to an Instagram for Mac app along with Carousel.
InstaDesk provides a slick and simple way to view Instagram photos of others. Note that you can only download Instagram photos - you can't upload them due to Instagram policy. But InstaDesk is still an excellent app suggesting new friends and people to follow based on the images you like. In addition, you can also comment on pictures, like them, download them, or watch them in slideshows.
InstaDesk is great for simply viewing your Friend Feed, following Popular Tags, Popular Pictures and commenting on them. It will also notify you when you have new comments or likes on your pictures. Of course, you can download images but you can't upload them.
InstaDesk is about as close to Instagram for Mac as you'll get - it's just a shame you can't upload.