Everyday, Gmail makes desktop mail clients feel a little more dated. The latest Labs features now preview stuff from YouTube (no more Rickrolling??), Flickr, Picasa and Yelp, and you can undo sending regrettable email.
Undo send, sadly, only has a five-second window to take it back (you can boost it to 10), so you better have excellent reflexes. Previews work a lot like the image preview you're used to. Here's the YouTube preview in action: