Firefox only (Windows/Mac/Linux): Blank Canvas is a handy little signature manager that automatically inserts HTML signatures into your Gmail messages based on which email address you’re sending from. The extension supports up to four unique signatures for each address you’ve designated. Signatures are automatically inserted in both new messages and in replies and forwards. If you’d prefer to use a Greasemonkey script to manage your signatures instead of an extension, make sure to check out Gmail Multiple Signatures If you have another technique for managing signatures—Gmail or otherwise—we’d love to hear about it in the comments.