Web: Instagram is loads of fun, but it doesn't provide the best interface on the web. Pictacular fills this void with a powerful, Pinterest-like layout for your Instagram feed.
Much like the web version of Instagram, you can like and comment on photos with Pictacular. However, Pictacular's interface makes the Discovery, Popular and Nearby feeds much more accessible. You can see full lists of your individual followers through Pictacular as well.
Pictacular's web version enables you to browse through photos in your feed with your keyboard. It also enables you to access Feeds, Nearby and search (unlike Instagram's own website). Unfortunately, you still can't upload a photo with the app (but how often are you doing that from your desktop anyway?).