expired Better Rename 9, $1.99 (Was $19.99) Mac App Store
This was posted 10 months 5 days ago, and might be an out-dated deal
This is probably the best file renaming utility for the Mac, if there's a better one please let us know. On sale just for one day, today.
As noted in the comments, Automator which comes with the Mac can do renaming of files in addition to other useful tasks. A good intro video to Automator http://www.youtube.com/watch?v=5R9tWdAlpFM