iOS/Android/Web: Instagram is loads of fun, but doesn't provide the best interface on the web and doesn't provide one at all for the iPad. 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 web site). Unfortunately, you still can't upload a photo with the app (but how often are you doing that from your desktop anyway?).