Audacity is probably the best free sound editor you can find, and now you can squeeze it onto your USB drive and take it wherever you go.
While we find the interface a little hard to get around, it's a really powerful and flexible application. Importing sound files is easy, they can just be dragged into the window, and then you can edit and manipulate it in many ways.
One basic useful tool for this is making tracks louder or quieter. You can alter the base volume of tracks in iTunes, but the actual file is not affected, and the end result is many files that play at different volumes. With Audacity, you can tweak the volume of the track so it's at a similar level to the rest of your library however loud it is. You can also use it to add fade ins and outs, cut them out altogether and add a variety of interesting effects.
You can also use Audacity to record live from whatever input you choose, and it's really easy to edit and play around with your results. Recording a speech and then adding background music is really logical, for example. Open the speech file and music file out together, edit the sound levels then merge together.
There are tons of other ways to use Audacity, it really depends on your creativity. This portable version does everything the original does, and simply requires installation onto your portable drive to run on a PC.
Audacity is great, and it's fantastic to be able to run it from a USB drive