Text and image based email signatures are currently supported and can be created on the Your Email Settings page.
Signatures are added automatically when composing messages, and can be edited before sending. When used in conjunction with Email Templates, signatures are appended below the template.
Note: signatures don't currently allow for the use of dynamic template variables.
To setup a signature, navigate to your Settings > Your Email Settings:
When composing a message, you'll see your signature appended to the message automatically:
You can even add pictures to your signature!