Another Audacity vote - but do you need a plugin to work with mp3s I seem to recall?
If all you want to do is split the file or edit the length, use one of these (or mp3DirectCut), not Audacity. These apps will work on the mp3 frames directly, whereas Audacity will decode the mp3 for editing purposes and then re-encode it, which means you'll lose quality through lossy>lossy encoding.