Adding a Rich Text / HTML Signature in Gmail was not natively possible before. So people had to download browser plugins or use different Greasemonkey scripts to make those ‘shiny’ signatures to work.
But thanks god that Google has finally listened to their users and started supporting Rich Text Signatures out of the box in Gmail. Simply go to your Gmail Settings page and the option will be right there for you. You can now customize your signature by adding pretty formatting, links, and images — or decide to leave things nice and simple.
The feature has also been added to all Google Apps users, so if your company has been using text based signature all this while – It is time for you to move to a more creative version of email signature!
Gmail now also supports a unique signatures for each email address that you have associated with your account. So you can use a different signature for each of the accounts that you use to send out emails from. On the Gmail Settings page, you can edit the signature for each account by changing the email address that appears in the dropdown menu.
[Source: Official Gmail Blog]