Try out these and other experimental features from the Labs tab under Settings:

YouTube, Picasa, Flickr, and Yelp previews

Instead of just links, see previews of photos, videos, and reviews right in your email.
Undo send
Oops, hit “Send” too soon? Give yourself a grace period of a few seconds to cancel sending, then edit your message before sending again.
Tasks mobile
Take your to-do list everywhere you go. Just go to mail.google.com/tasks from your mobile browser
Offline
Make Google Mail work even when you’re not connected to the internet.