I commonly use qmv (from renameutils):

qmv, qcp - Rename or copy files quickly, editing the file names in a text editor

Found nothing better for bulk renames because you get to use all the pattern/replace power of your favorite editor instead of learning some wierd specialized grammar through trial & error.

This is how to remove all spaces from mp3 filenames using vim:

qmv -f do *.mp3

:%s/ /_/g

:wq

for Emacs users, there is wdired mode, which makes a directory listing editable just like a normal Emacs buffer.

A lot of what I find lacking in a standard terminal, Emacs makes up for with its shell and terminal emulators. I find that an editable buffer with the contents of my entire session to be eminently powerful when combined with keyboard macros and the search/replace commands.

Has anyone had any luck getting this to compile for OS X (Leopard)? make seems to fail on an error for me.