Flash Renamer 4.6
Although its not freeware, its not hard to find cracks/serials for it. Not that I would know... ;)
Anyways.
I use this one on a regular basis, whether its renaming files, or folders, alphanumeric, adding, cropping, randomizing... etc.
It does everything that i need.
http://www.rlvision.com/flashren/about.asp