From the terminologist toolbox: Lupas Rename
You only need to download the .exe file and upload a selection of files. Then you only need to choose if you want to fully change the name of your files or just add a label before or after your file name. You can also number them!
You can download it for Windows here: http://download.cnet.com/Lupas-Rename/3000-2248_4-52374.html
For MAC alternatives visit this page: http://formac.informer.com/lupas-rename
Watch this 3-minute tutorial in English: https://www.youtube.com/watch?v=3jVm-z05sBM
Or this 2-minute tutorial in Spanish: https://www.youtube.com/watch?v=_4CEmnBRVPY
Also, if you are a power user and want more options, here is a great post by Gizmo’s Freeware that provides other free file renaming tools.