MP3 Tag for editing tags and renaming to some extent
MP3 Gain for adjusting perceived volume on playback
Name It Your Way is a fantastic bulk renaming program. It could change a thousand files named like this:
Elvis - Mystery Train.MP3
Elvis Presley - Mystery Train.mp3
in about 2 seconds.
Encspot is a great little app to analyze your mp3 collection and provide details on the encoder used, file size in bytes, bitrate, stereo mode, running time, etc.
MP3 Utility can scan your mp3 directories and identify those files that have faults and may play poorly. It can also print out a text file of all mp3s you have.
MP3 Direct Cut can edit mp3s, cut off noisy tails, noisy intros, etc.
Audacity is a fantastic recording/playback/editing application that has all sorts of tools to modify mp3s--fade in, fade out, boost certain frequencies, edit out clicks and pops, reduce tape hiss or groove noise, etc.
Switch is a good application to convert sound files from one format to another.
All of these are free and easily found on the Internet.