Making all MP3s the same volume

What's the best Mac app to make the peak volume of my MP3s all the same?

i.e. I set a maximum volume, and the software tweaks all my MP3s so that the loudest part of the track matches it.

I listen to my music when I commute on my motorbike, so I need a consistent volume across all my MP3s, as I can't keep changing the volume whilst riding.

At the moment, I can't hear some tracks, whilst others make my ears bleed. :p