Métamorphose File -n- Folder Renamer is a free, open source mass renaming program that combines great flexability with an intuitive interface.
Allows many command line renaming operations in a single utility, perfect 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) will not find it too frustrating to use.