HTML Signatures in Gmail

Now one can create HTML signatures in Gmail and insert them using a Greasemonkey script: User Script 


In Gmail, it creates a box (shown aside) where one can create a signature.





Firefox Add-on: Black Canvas Gmail Signatures

In their own words,
Automatically inserts HTML signatures into your Gmail messages based on which address you are sending from. Supports up to four signatures for each address you have set up. Works for Compose Message and Reply/Forward.