It's beginning to add an unsubscribe button to Gmail that will automatically appear in the header of emails that it detects might be unwanted messages from marketers.
I'm pretty hard on Google but this is a legitimately good feature. If I were cynical I would say they're letting you unsubscribe from spam they helped you get by giving out your email address to advertisers. There isn't any evidence of that at this point so I'll take this as face value. A good feature meant to increase the value of Gmail to its users.