Rename Multiple Files is a free and very easy to use Automator based workflow which will alllow you to quickly rename any number of files you selected in Finder.
Rename Multiple Files can also be used as a system service by Snow Leopard users.
To do that you will have to follow the following steps:
1. Copy the workflow file in the downloaded archive to /Users/yourname/Library/Services/ (if 'Services' doesn't exist, create a new folder and name it likewise)
2. Open a Finder window with multiple files. Select all of the files. Right-click on the selected files. At the bottom of the contextual menu you will find an option titled 'Rename Multiple Files'.