Last Thursday, Gmail introduced an "undo send" email feature that gives users 5 seconds after hitting send to recall virtual gaffes. Users can enable the feature in the "labs" section of their account settings.
Reuters reports on how Google is attracting users and working to take ad revenue and market share away from Yahoo by introducing features like this at a "dizzying rate." According to the article, since Gmail labs launched in June, Google engineers have created 34 different opt-in experiments, like "mail goggles", which aims to prevent users from sending drunken (and potentially regrettable) messages by requiring them to do math problems before emailing during certain times like weekend nights (users can tweak the schedule).
Engineers receive direct feedback from users on a discussion board which, according to the article, helps speed feature development. Feedback on the unsend feature includes some requests for Gmail to hang onto emails for longer than 5 seconds. According to CNET, Google testing found that 5 seconds was a good compromise between allowing recall and introducing too much lag time to email conversations, but they may introduce longer wait time options in the future.