mvsed is a script that will rename files in bulk by way of regular expressions.
I had a large number of media files (no, not pirated media; my own photos) that needed to be renamed after hitting the 10,000 photo mark, and as such, overflowing the 4-digit photo number allowed by the DCF specification that nearly all cameras comply with.
You can get the initial release of mvsed here. The download is approximately 6kb. It requires Python 2.5 to be installed.
If you have any comments or queries, email me at mvsed at this.domain.org.
mvsed is released under the BSD license, as per the comments included in the script.