The Digital Inspiration weblog has a step-by-step for adding
HTML to Gmail messages.   Personally, I'm not a huge fan of
embedded anything in emails (give me plain text, please!), but
for those of you who have been missing HTML in Gmail, Digital
Inspiration's method should do the trick. If that method isn't
your cup of tea, you might also want to try the Gmail Skins
Firefox extension, which allows you to embed HTML directly in
Gmail.