If you've got several Gmail accounts and are frequently having to juggle signatures for each of them, worth downloading is Blank Canvas' Gmail Signatures. This experimental Firefox extension will drop in one of four custom HTML signatures based on whichever account you're sending the message from. If you're like me and have two or more accounts, setting this up is a big time saver.
Once installed, you get a new drop-down menu that lets you select one of your four custom signatures. These can be managed directly within Gmail, and come with an editor that shows you a live preview of whatever HTML you drop in. Included are four presets with nicknames like personal, business, and family, all of which can be renamed to suit the type of signature you've set up.
It's worth noting you cannot get at your custom signatures on browsers without the extension installed (even if it's the same machine), and this will not change existing Gmail signature settings. This means that any Gmail-specific signature you have will still show up, however, they'll appear underneath the one from the extension.
Gmail Signatures is an experimental add-on, and as such you must be registered with Mozilla's Firefox add-ons site to download it.