This excellent little utility (formerly known as File List) can help you rename multiple files at the same time, in just about any way that you can imagine--for example, if you want to name a bunch of photos in a sequence or change some PC files from ugly all caps over to lowercase.
Name Mangler gives you a clean, intuitive interface for numbering sequentially, finding and replacing, changing case, setting extensions, adding prefixes and suffixes, and removing and inserting characters. Name Mangler also offers an Advanced option that lets you define your own renaming rules, combining the standard options and letting you describe new file names as expressions--and the ability to use regular expressions gives you almost limitless possibilities. This app even lets you save configurations as droplets, so you can execute the same set of renaming rules again and again with a simple drag and drop. If you need to frequently rename files, Name Mangler is a good value, and you can try it for free for the first 25 launches.