It is free, but apparently not "open source", and has a host of useful features. Adding your "suffix", as the utility calls it, is a piece of cake for multiple files. It is only available for Windows and it can be run from a USB flash drive. There is even a command line version for batch files and shell scripts. (But there is not a DOS version.)
Home page:
http://www.bulkrenameutility.co.uk/Main_Intro.php
I use it to maintain the "file naming guidelines" on the shared drive. Of course it drives the users crazy if they don't follow the guidelines and then make link files in other places before I correct their "real" file's name.