Protect your Gmail from Spam
If people from your address book are getting nasty spam emails from your account, chances are high that your email account is phished and you are not aware of it. Google has introduced a two way authorization system to avoid this. This 2 step verification process requires that, in addition to your password you need to supply an additional passcode sent to your mobile (Similar to internet banking used by many banks).
Read more about this feature in this blog. The official announcement of this feature in the google blog can be found here.