If you want to unsubscribe…, click “show details” in the top-right corner of the message, then click “Unsubscribe from this sender.”
GMail will then send the unsubscribe request for you. This only works for legitimate listservs. For spammers, do just that. Mark it as spam.
Unsubscribe.com does more that what GMail does, and will unsub you from most any mailing list. Their Unsubscribe for GMail add-on has one-click unsubscribing with 5 free per month or unlimited for $19.95 per year. It also works on Yahoo, AOL, and Hotmail.