Flexible Renamer
Flexible Renamer is perhaps the most singularly useful program I've found in a while. It's capable of sorting, renaming, copying and sorting files based on numerous different ways of searching and organizing them, including regular expressions and wildcards. So, you have a ton of files that need renaming or sorting by category, it's pretty amazing.
It also has the ability to read things like ID3 tags and rename your entire MP3/WMA collection by artist, title, or anything else.