Here's a Gmail feature I wouldn't have noticed without a post from Gmail's blog: emoticons. If you use the rich-text mode when composing messages, you can now include small images that reflect your intentions.
"The black and white days of text-based emails have had their day. Following the evolutionary path blazed by colored labels, we present, in all their technicolor glory, emoticons in your mail," says Darren Lewis from Google.
Google also added a new set of emoticons in Gmail Chat and it's strange to see that you can add much more emoticons in an email than in an IM conversation.