Most iPhone users are familiar with this wonderful app. It offers several different "film" types and lenses that distort in a colorful, exaggerated way. It's fun and makes everyday snaps look artsy and retro. Some of the Instagram filters seem to mimic the look of Hipsta, but the real deal is still the king for funky street photography.
Old Photo Pro (free)
One of several apps that give images that turn-of-the-20th-century faded photo print look. Not only does it give the pictures a weathered texture, but the edges also have a burned and damaged look.
One of many apps that simulate the shape and look of a Polaroid Instant photo, but taking it one step farther: When you click the shutter to take a photo, it actually makes the "click . . . whirrrrr" sound of an instant picture exiting the Polaroid camera. And if that's not cute enough, you actually have to shake your phone back and forth for 30 seconds to develop the photo, just as in the old days. I like the old Polaroid look, but I'm still deciding whether I find it silly or clever that the designers went to all that trouble.
6x6 (99 cents)
I post to Instagram, Twitter and Facebook using the functionality of Instagram. But my preference is to take the original photograph using 6x6, which keeps the square Instagram dimension but doesn't force me to process and upload immediately. It lets you shoot and automatically save to your Camera Roll. It also gives you the option of color or black-and-white, it offers a grid so that your horizons don't tilt, it gives you the option of manual focus and exposure control, and if you prefer, it doesn't add additional processing to your images.