There is ihearclear and amplitude, both by gripwire.com.
I got ihearclear, and while not as refined as dedicated hearing aids, it's a great price at only 99 cents. I have a pair of hearing aids I haven't taken out of my drawers in years because since I stopped going to school, I just don't often find myself in a situation where I have to listen to people talk, and I get by ok in everyday conversation without them. But occasionally, I like having sounds amplified, like say if I go see a movie. Now I don't have to hunt around for my hearing aids when I go to a movie. Or get caught without hearing amplification if somebody asks me to a movie on the spur of the moment. I always have my iPhone or Touch with me wherever I go, so I just whip it out, turn on the app, and I'm set!