After seeing a post on Lifehacker talking about Gmail's new, or at least newly discovered, Auto-Unsubscribe feature, I had to try it.
Basically it works like this: you get an email in your inbox that comes from some kind of mailing list, and you hit the 'Report Spam' button. For certain lists, you'll get a pop-up window offering to unsubscribe you from the list. I decided to test it out.
The first challenge was finding a likely candidate. I kid of course...it took me about three seconds to find a mailing-list email that I didn't want and had never bothered to unsubscribe from. I opened it and clicked Report Spam and sure enough, a window popped up saying:
I clicked 'Unsubscribe and report spam' (and it was spam, I've never opted in to a CompUSA mailing list) and a few moments later an email with the subject 'CompUSA.com Unsubscribe Confirmation' arrived. The mail came from CompUSA, not Gmail, so results will vary depending on how gracefully the company in question handles unsubscribe requests.
The process worked quite quickly for me, but Google's help page warns it could take a few days. They also take care to point out that they won't try to unsubscribe you from lists from known spammers (since doing so just makes your address more valuable to a true spammer). See the Gmail Auto-Unsubscribe help page for more details.