Bulk Renamer

Bulk Renamer (MS Windows Only):
http://www.bulkrenameutility.co.uk/Main_Intro.php

This is another file renamer.  I like this one a lot because it does things that other renamers don't.  At least that's what I remember when I was searching for renamers.

NameChanger

NameChanger (Mac OS X):  http://www.mrrsoftware.com/MRRSoftware/NameChanger.html
Free software designed to help batch renaming of file and folders quickly and with ease.  Among various renaming options, the software supports regular expressions, mismatched files types, and formatting changes.

Lupas Rename 2000

Lupas Rename 2000 (MS Windows): http://rename.lupasfreeware.org/lupasrename.php
Simple and powerful program to help rename file and folders.  In addition to being free, the software is just a simple ".exe" that does not require installation, so it is small and extremely portable.  Perhaps one of the programs that should be handy on a USB drive.