EasyHearingAid from Link Advance&Innovation
EasyHearingAid is steeply priced at $14, although it does give the user a lot of control over the sound, with individual sliders for 13 different frequency ranges. Despite its name, the app claims not to be a hearing aid, perplexingly, and its description says it's intended for people just beginning to lose their hearing.
Kikitori from Grace Co. Ltd.
Definitely the high end of the market price-wise, it's hard to imagine any number of features justifying Kikitori's $35 price tag. The app (Kikitori means "good hearing" in Japanese) has a lot of sound adjustment controls and a noise filter.