If you haven't already seen it, thanks to Coloribus for creating a link to the first One second commercial ever. Yes, it's literally only one second long. Adrants reports that "One Second breath freshener placed a once second commercial during every commercial break on every TV station in Belgium" in May of 2005.
Watch it here
Update: Thomas K points out here that the first one-second commercial was created for Master Lock in 1998. Thanks