MailTunes for Mac

From Lighthead Software:

With MailTunes, you can have email signatures that changes according to your rules. For example, it can display what music you play in iTunes, or the uptime of your computer. You simply compose a signature in MailTunes which contains magic keywords. When MailTunes updates your signature, those keywords get replaced by the appropriate information.
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.