Google has rolled out rich text signature support to Gmail so you can have a fancy signature without any Greasemonkey scripts or Macgyver-like cobbling together of Google Labs features.
If you hop into the Settings page of your Gmail account you'll see a new signature section with some familiar rich-text editing buttons and options. You can tweak your text size and colour, insert images and links, and otherwise format your text as you can within a rich-text Gmail message. In addition to rich text signatures for your primary address if you have any custom From: addresses set up you can create a custom signature for each address. Thanks debobleve!
Rich Text Signatures [The Official Gmail Blog]