I'm very excited about Google's new two-factor authentication. Two-factor authentication means, in this case, that instead of just authenticating based on "something you know" (your password) you also authenticate based on "something you have."
In this case, the "something you have" is your mobile phone. Once this is enabled, you give Google your mobile phone number. Then whenever you try to log on to your account, Google sends a SMS message to your mobile with a six-digit number you enter to prove that you are really you. (There's a "remember me for 30 days" checkbox to make this non-awful on your own computer, which is, after all, "something you have").
How does this interact with IMAP and POP3 clients? What about accessing Gmail through the API?