Script Summary: Allows you to enter custom signatures for each From: address in Google Mail (gmail).
But, it doesn't allow you to have different signatures for each of these emails.
Gmail Dynamic Signatures allows you to create custom signatures for each e-mail, stores them in the browser, and pops them in intelligently based on the e-mail address you're using.
When you change the "From" address from the dropdown menu, it automagically updates the message body with the signature you assign to that address. You can also set up a default signature that is automatically added for all addresses without a custom signature.
JSLinted for additional self-punishment.