Snoop-Kitty from Sierra Software
Snoop-Kitty has only a gain adjustment, but it does add a cool factor, incorporating an oscilloscope for viewing waveforms as it takes in sounds and rebroadcasts them through the headphones. It goes for $1. The name suggests a secondary use case many of these apps flirt with in their descriptions: eavesdropping.
Another basic offering, for $1, i-Hear has gain and volume adjustments, and an on-or-off noise filter. Once again, the developer offers a free app as well with no clear difference in functionality between the two.