The proggy I've always used is MP3 Rage (current version 3.5 at
http://www.chaoticsoftware.com/ChaoticSoftware/). I really like it.
Thing is, it's not free, but it does just about every MP3 function
you can think of; I quote from the web site ...
"MP3 Rage is the definitive collection of tools for MP3 enthusiasts.
You can search for and download thousands of MP3's from the Internet
using the Napster(tm) music sharing system and the Gnutella file
sharing system, edit ID3 tags, analyze MP3's and adjust their sound
volume automatically to play at similar levels, export MP3 files to
AIFF format for burning CD's, play MP3 files, catalog, organize,
rename, find, and change thousands of MP3 files and their ID3 tags
quickly and painlessly. It does all this with a clean, powerful, and
easy-to-use interface."