If you have ever been frustrated with the signature limitations of Gmail, then you will want to get this plugin. Blank Canvas is a nifty tool that has developed from a greasemonkey script into something that has quite possibly saved the life of my poor innocent laptop on more than one occasion. The plugin gives you the ability to have multiple signatures in Gmail that can be tied to particular send-from addresses and to the contacts you send to. You are able to set up categories, as well (the defaults are Default, Business, Family and Friends). This is a particularly useful tool for people who, like me, have several email accounts that are all directed to a single gmail account.

Added bonus... Fully HTML compatible. Yep, that means you can have custom images made up, host them, and call them in your respective signatures. Very nice if you want to add a bit of branding to your message!