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Not sure why this has to be so hard - In Windows, you just highlight the files, right click, select rename and type in the name. wow. 15 seconds.

WHy should I have to buy some software or run scripts? I thought Mac was supposed to be user friendly...
 
Although I've tried and used Automator, I find my Canon SLR DPP software can batch rename my photos most efficiently per my image import/editing workflow.
 
Name Mangler http://manytricks.com/namemangler/ is free if you can put up with a nag screen every now and then. It offers many ways to rename files. I keep it on the dock and just select the files I want to rename and drag to the dock and drop on Name Mangler. You can see the results before you are committed to it, also you can select where you want them saved to if you don't want to over write the originals. Name Mangler is also good for doing a Search and Replace inside of files and folders.
 
Name Mangler http://manytricks.com/namemangler/ is free if you can put up with a nag screen every now and then. It offers many ways to rename files. I keep it on the dock and just select the files I want to rename and drag to the dock and drop on Name Mangler. You can see the results before you are committed to it, also you can select where you want them saved to if you don't want to over write the originals. Name Mangler is also good for doing a Search and Replace inside of files and folders.

Then there's NameChanger, which is completely free, so it has no nag screen. :p
 
I created an Automator Service similar to what fatralph described called "Batch Rename Files".
Batch%252520Rename%252520Service.jpg


Since it's a service, just select all the files you want to rename in Finder, then right click>Services>Batch Rename Files
 
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