Name Mangler 2.0
If you've got a bunch of files that need renaming, Many Tricks' Name Mangler 2.0--a major update to a previous Mac Gem, File List--is my favorite tool to use. You just drag your files into its window, choose your renaming options, and click a button.
The range of easy-to-use renaming options includes find-and-replace, numbering, case-changing, extension-setting, adding prefixes and suffixes, and removing and inserting characters.
There's even a mode for using a special scripting language for more-advanced procedures. A live preview shows you how each file's name will be changed before making the change.