Clean up file names using Automator
There is no need for dedicated file renaming software to do such relatively simple batch renaming operations – Automator will allow you to create services that do what you want.

Your first task (removing a numerical prefix followed by an underscore form file names) can be solved like this:


create an Automator service (choose “Service” when prompted what kind of workflow to create by Automator). Set it to take “Files or folders” as input.
add a “Filter Finder Items” action and set it to filter on the condition “none” match “kind is Folder”  to make sure you have only files.
Add a “Run Shell Script” action and make sure it runs /bin/bash and takes its input “as arguments”. Insert the following code:

shopt -s extglob
for f in "$@"; do
    fpath="${f%/*}"
    fname="${f##*/}"
    mv "$f" "$fpath"/"${fname/#+([[:digit:]])_/}"
done


– see the Bash Reference Manual’s chapter on Pattern Matching for an explanation of how this works (recommended reading for this kind of operation anyway :)).
save to the default location as “Remove Prefix”.


– you will now have a “Remove Prefix” in the Service menu of Finder (and the context menu of files and folders) which will rename files so a numerical prefix followed by an underscore is stripped from the file name.

The second task (prefixing the files with a selectable string) is even easier to achieve, using a Standard Automator action:


create an Automator Service as in the steps 1 and 2 above.
add a “Rename Finder Items” action. Set it to “Add Text”, enter a default prefix in the text box (if you like), set the string to be applied “before the name”. Select “Options” in the bottom row of the action and tick “Display this action when running” (disclaimer: I’m on a German system – all translations approximative).
save to the default location as “Add Prefix”.


– when you run the service on one or more files, you will see the following dialog:



(or rather, its equivalent in your system language), where you can edit the prefix you want to use before applying it.

Happy renaming with OS X’ on-board abilities :).