Batch Renamer is a powerful cross platform tool that allows you to rename several files at once.

Batch Renamer screenshot.
Download the latest version here (please review the license below):
Key features:
For more information, please see the user guide. Additional information about regular expression syntax can be found at the Regular Expression Library.
Get involved:
You can help this project by:
Support this project - If you use Batch Renamer, please consider donating to support the project.
The Subversion repository is hosted at opensvn.csie.org. You can get the latest trunk version by running:
svn co http://opensvn.csie.org/renamer/trunk batch_renamer
External link:
License: Batch Renamer is licenced under the GNU Genaral Public License (GPL). Basically, it means that this software is freeware, the source code is available and (quoted from GPL) "this program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY".