The story of the Cow Level is one of the oldest jokes, or myths, or rumors, about Blizzard's wonderful game Diablo. There are three cows on the village map. If you click on them, they'll moo; if you click on them repeatedly, you'll get some funny quotes from your hero. But rumor soon spread that if you clicked on them often enough, you would be taken to a secret place: the cow level. As time went by, people thought of more and more absurd ways that would allegedly take you to there.
Some would try to denounce the myth, but many others upheld it and would go to considerable lengths to make it plausible. In May 1997, there was even a fake screenshot. I think I have finally found it. It is well crafted, though the array of monsters is a bit strange. Ever seen Fallens and Witches on the same level?
And so it went on. Then, I think it was in 1998, someone even bought the domain cowlevel.com and dedicated a whole site to this phenomenon. It has vanished into the depths of cyberspace by now, but you can read the ways they suggested here. The design of this site here is inspired by it, too; that's why it is so difficult to read, even though I chose another font and brightened the contrasts a bit. Mozilla users can view it in an even more original and unreadable style, or without the stylesheet; Opera users can turn it off, too; IE users, as usual, are doomed.
So widespread was the myth that when Blizzard created Diablo II, they really inserted a cow level, as did Sierra in their add-on Hellfire. So if you really want to battle the horny lactating beasts, here is your chance.