SigPro gives you the ability to create a signature once and use it for all accounts, even if it has account specific info in it, like your email address. Elements that can be added generically include the email address, username and account name. Using SigPro, you can insert whatever text you want into your signature by calling a script at the moment the message is created.
Mia for Gmail (was Notifier Pro for Gmail) is a desktop Gmail client which sits politely in your menu bar.
