ToneTester is a Mac OS X Cocoa application that tests hearing. Specifically, it looks for offsets in how each of a person's ears hears pitch (the formal term for this is "binaural diplacusis" or "interaural pitch difference").

You can find the ToneTester application here (Mac OS X 10.3.9 or later, including 10.5 Leopard).

You can find the source for ToneTester here (XCode 2.5 or later, Mac OS X 10.4 or later).