The Greatest Easter Eggs in Gaming

GameSpot has published an article entitled "The Greatest Easter Eggs in Gaming", in which they detail classic hidden game features like the secret cow level in Diablo II. A little something to start you off:
Diablo II was a serious game, but it didn't mind exhibiting a little humor now and then. Once a rumor surrounding the original Diablo, the infamous "Secret Cow Level" was a reality in Diablo II, and infused the strategy game with a little fun. A specific sequence of events and one complete play-through of the game let you open a portal to a world where cows reigned supreme. All you needed was the leg of Wirt (an artifact from the game's first act) by the time you defeated Diablo in the game's last act. A trip to town and a transmute later and you'd have an open portal to a place known only as the "Secret Cow Level," which was filled with a seemingly endless amount of hell's bovines. Of course, the sheer number of these creatures and their high levels provided a worthy challenge to Diablo II fans, who now play that level almost exclusively to power up their characters. More humorous than the fact that you're fighting hordes of two-legged cows is that they emit quite-human-sounding "Moos," and they literally swarm you. Once they finally let up, you can fight the Cow King, but defeating him means sealing off the Secret Cow Level forever. So do so with caution. The Secret Cow Level is an example of an Easter egg that takes some work to unlock, but it also provides a decent reward. Not only does it exhibit the humor of Blizzard North's programmers, but also it's worth the effort financially.