Today I converted all my mp3 files to ogg !
Ogg is an audio format which is superior to mp3 in quality and compression, and most importantly it is open.Visit http://playogg.org for more details on ogg format.
Wrote a small script for doing that :
OGG=`dirname $MP3`/`basename $MP3 mp3`ogg
if [ $# -lt 1 ]
echo "usage: mp3toogg input.mp3";
mpg123 -w - $MP3 | oggenc -o $OGG -
though it took a whole day to convert 10 Gb of my music, I saved 2 GB of space when it was finally done.