GMail has a new feature called Undo Send. You have 30 seconds to undo a sent email because GMail will hold the email for about 30 seconds before sending it out. This gives the sender plenty of time to ponder if the message was a good idea or not.
To use the feature, you will need to enable it. Click the Green Labs icon in the upper right corner of the GMail screen. Scroll down and enable the Undo Send feature. Give it a try. Send a message to a friend and try to pull it back before 30 seconds has expired.