N.Y. Times: “Make Several Gmail Addresses Out of One.”
Overall, this is a great technique to help weed out spam or filter legit messages, but it’s not without its issues. Some years back, for example, I subscribed to a game company’s email newsletter using this method. When I decided to unsubscribe, I had to enter the email I used, which included the plus; the website rejected it as invalid. Long story long, buyer beware.