How To: Have Multiple Signatures in Gmail with No Extensions, Userscripts or Bookmarklets Required - The Next Webby groucho
1. Enable Canned Responses in “Labs” in your gmail account.
2. Write out your email signature into your email as if you were writing a normal email.
3. Next save the signature as a canned response under any name, something like “My Main Signature” for example. (repeat this step for all your various signatures)
4. From now on, you can write emails as your normally would and once you’ve reached the foot of your email and ready to insert your signature - simply click the “Canned Responses” menu and select the appropriate signature.
The downside? You unfortunately can’t insert HTML signatures.