I hate it when some songs playing in iTunes are so much louder than others. iTunes’ Sound Check helps, but not enough. iVolume is the answer. The Techno Geek Speak:

iTunes uses a very fast algorithm to quickly analyze the volume of its songs. It’s fast, but it’s not very accurate. iVolume corrects the volume information that iTunes has calculated with better values. Therefore an algorithm is used that is oriented on the perception of the human ears.

In other words, using iVolume makes music in iTunes sound better, even if it isn’t.