NameChanger is designed for the sole purpose of renaming a list of files. And its doing its job pretty well. You can replace characters, add numbers or strings even use regexp and access the exif data of images.
Add the files through the GUI or drag and drop files directly onto the App. These original filenames show up in the Original Filename column. Grayed out rows indicate files that you do not have permissions to change.
Next select how to change the file names:
•Replace First Occurrence
•Replace Last Occurrence
•Replace All Occurrences
Download here for free (if you feel like it donate to the creators)