There are tons of photography apps out there, and most of them are crap. This is not one of them. Instagram is free, easy to use and has a variety of filters for photos to experiment with. It is extremely popular on Tumblr.
The final photos produced by Instagram are square, but the original, unfiltered photo is still available in the photo roll (for future editing). There’s also an option to import a file from the photo roll and apply a filter to it. As you can see below, there’s a private feed for your own photos and a public feed.
From the app itself, you can title, geotag and upload the photo to Twitter, Facebook, email, Flickr, Tumblr, Foursquare and Posterous.
A recent update has added a tilt-shift function so if you’re into depth of field manipulation (DOF) you are in luck. Try it out, it won’t cost you anything.