Any recommendations for a freeware file renaming utility?

I get a lot of audiobooks on CD. However, in order to get them to properly show up under the audiobooks section in iTunes and have the bookmarking feature, I need to manually change the filename from m4a to m4b.

I'm looking for a super quick and easy utility for this, preferably something that allows me to save a "default" folder where this stuff is usually stored, and save a script to change extension from m4a to m4b.

There seems to be hundreds of these utilities out there, so I was hoping somebody here might be able to recommend a good one.

Thanks in advance.