I always wanted to attach a html email signature to Gmail, but by default gmail does not allow html email signatures (why?). But if you combine the power of greasemonkey with Firefox, you can add html email signature to Gmail!

Greasemonkey is a powerful Firefox plugin that allows you to change how your favorite pages behave and look. If you power up Firefox with Greasemonkey and then add the appropriate scripts, you can modify the functionality of any web page.

After enabling Greasemonkey, get the Gmail Signature Float userscript. It will modify gmail in 3 amazing ways –

Moves your signature in Gmail to the top of the quoted message rather than the bottom. I still wonder why gmail attaches the signature at the bottom of forwarded or replied messages. This fixes that issue.
Secondly it can strip out the two dashes that Gmail automatically places above the signature. If those 2 dashes irritated you, they are gone now.
Thirdly, It enables the ability to add HTML to signatures, and the ability to leave turn off and on “floating” of the signature.

Of course there is the trick of using the Feedburner Email Headline Animator in Gmail via using desktop email clients like Outlook. If you want a more powerful gmail experience, Gina Tripani at Lifehacker has compiled several Gmail Greasemonkey scripts (including the Gmail Signature) into a cool firefox extension called Better Gmail, (now listed in 50 best firefox extensions) which adds a long list menu of optional extra features to Gmail worth checking out.