This Chrome extension lets you compose emails in Markdown, and then convert it to HTML to use in Gmail. Gmail doesn’t offer you much in the way of formatting to make your emails look more attactive.

Enhance your emails in different ways. Hyperlinks, numbered lists and quotes can be used. You can show your feelings with emoticons and highlight key passages in color or bold face. Use the HTML mode ...

I suppose you already know what Markdown* is. For those who don’t know, let’s quote a few paragraphs from John Gruber’s Daring Fireball: Markdown is a text-to-HTML conversion tool for web writers.