Developer website: Lighthead Software
If you want to use information other than iTunes info, you can write your own AppleScripts or shell scripts to fetch this information and return it to MailTunes. The signatures gets appended to your mail messages, just like ordinary mail signatures. The signature updating is done in the background, and can be set up to automatically start when you log in. MailTunes works with Apple Mail, Microsoft Entourage and Bare Bones' Mailsmith.