I haven't used this yet, but it's a beast!
"You can rename files, substitute some or all of the file name, apply numbers and lettering, swap extensions, append with time and date stamps, rename based on image EXIF data and audio ID3 data, and all that just scratches the surface." - Lifehacker
I guess it also has a quick preview feature, to make sure everything will turn out the way you wished. And it's freeware.
http://www.bulkrenameutility.co.uk/