In Gmail, I have a number of accounts that I can choose to send from. (Old e-mail address, one used just for gaming, one for my blog, etc.)
How can I have a unique signature for each depending on which I choose?
Google just released Rich Text signatures, which also includes the ability to have a unique signature for each From address.
Gmail also now supports a unique signature for each email address associated with your account. So, if you send mail using a custom "From:" address, you can use a different signature for that address. From the Settings page, you can edit the signature for each account by changing the email address that appears in the dropdown menu.
Have you looked at canned responses;
These can be used along with filters.
Check out WiseStamp extension for Firefox and Chrome.