INK361 (formerly Inkstagram) is a web app for viewing Instagram images in your Mac or PC.
INK361 might sound like a boyband but the makers were asked by Instagram to change the original name of "Inkstagram". To log-in, simply use your Instagram username and password.
INK361 allows you to interact with your Instagram friends, photos and feed. It's main features are the ability to view your own photos, comment on and like photos, search by user, search by hashtag, follow people and finally share images via Facebook. Note that you can't upload photos to Instagram - only view them.
INK361 is not a perfect solution but in the abscence of an official Instagram app for PC or Mac, it's a good web based solution.