Siren is a utility that can rename multiple files simultaneously through the use of expressions. It can use the information associated with various types of files, like for example, the ID3 tag info from MP3 files, which you can modify to your requirements. It's a powerful program, with an intuitive interface that lets you find and select files that you want to rename easily. However, in using expressions the rename process can prove somewhat complicated, although you can easily find help for doing it correctly.