Métamorphose is a free, open source program to mass rename files and folders which combines great flexibility with an intuitive interface.
Allows many different renaming operations in a single utility, great for those of us that need to rename large numbers of files and/or folders on a regular basis.
However, the ease of the interface ensures a one-time user (I need to rename 5 gigs of mp3's) should not find it too frustrating to use.
Cross platform for your pleasure, it can be used on a variety of operating systems.
Tools and Utilities
Product page: here