I just might have to switch to Gmail solely because of this feature : Undo Send. Honestly, how many times have you sent a message and then wanted to kick yourself -- like forgetting to add an attachment, or hitting the send button on accident, or just having second thoughts. Another big mistake I have made is CCing to someone instead of BCing. Oops. In Gmail with a little feature from Google Labs you have five seconds to undo a sent email. Awesome!
Also this little feature from Google Labs, called Mail Goggles, made me laugh. Jerry Maguire, anyone? I wonder though, what if you're absolutely horrific at math? Could get sketchy! :)