Find the Invisible Cow
Move your cursor around a blank field; the cow moos louder the closer you get.
Exactly what it sounds like — an invisible cow is hidden somewhere on the page, and your only clue is a mooing sound that gets louder as your cursor gets closer. It's a genuinely old joke of a site (a proof-of-concept-turned-cult-classic) that has stayed free and ad-light the entire time.