Name Mangler: batch rename your files
Most people rarely ever have to rename a whole lot of files at once. Let's face it -- batch file renaming isn't UNIX kernel reprogramming, but it's one of those irritating tasks that can be a surprising amount of trouble for casual or even more experienced users without the right tools. As a web designer and developer, I find myself having to numerically rename images a lot or add "_tb" suffixes for thumbnails, for example.
Luckily for Leopard users, there's Name Mangler 2.0, a donationware app by Mac developer Many Tricks that does all the stuff you might need a batch file renamer to do: change case or extension, prefix or suffix a file name, remove characters, the works. You can even store renaming configurations as droplets that you can drop files or folders on to.
Unfortunately, Name Mangler is Leopard-only. Many Tricks has an earlier app, File List (direct download link), that apparently does the same thing for pre-Leopard systems; another alternative app is File Wrangler, which resides on my 10.4.9-running MacBook Pro quite happily, or the venerable and powerful A Better Finder Rename.