There's really nothing worse than sending out an important e-mail with mention of an attachment then forgetting to actually attach the file that goes with it. To that end, a brilliant new Gmail labs feature is an opt-in nagger that will give you a warning pop-up if you try to send without a file and have used the word "attached" somewhere in the message.
It's not a perfect system. For example, if you say, "I've put the files in this e-mail" or "included in this message" it won't warn you. Amit over at Puse 2.0 also noticed that the system managed to flub "here is the attachment" and "Attach a document," so the phrasing is not an exact science. With anything like this there's a careful balance between making the act of sending an e-mail sloppy yet simple versus precise and difficult.
To turn on this new feature you must opt-in through the Gmail labs settings menu.