A Gmail feature lets you unsubscribe from a newsletter or a mailing list when you report one of the messages as spam. "You'll see unsubscribe tool when you mark a message from particular types of mailing lists as spam. If the particular message is a misuse of a mailing list you like to receive, you can Report spam as usual. But if you never want to receive another message or newsletter from that list again, click "Unsubscribe" instead. We'll send a request to the sender that your email address be removed from the list," explained Google
Unfortunately, this feature doesn't work for all newsletters and mailing lists, so it's not easy to see it in action. I've noticed that "auto-unsubscribe" works for Google Groups messages and for Google's newsletters.
To find all the "auto-unsubscribe"-enabled messages, search for label:^unsub.