Price (older version): free, link
Price (newer version): $26, link

Renamer4Mac is a simple tool to batch rename your files. All you need to do is drag all the files to its window and set the rules. On the older version you can rename the files based on Search & Replace, Insert/Overwrite, Remove Characters, Number, or Uppercase/Lowercase. This tool is very handy if you do a lot of repetitive file renaming.

While I am writing this post I just realized that this app is no longer maintained by its original developer. So you need to spend an extra $26 (with 10 days free trial) to get the latest version. I am sure more features have been added, but you can still get the one I have been using all this time here.