Examples of Easter Eggs:
In Rise of the Triad, playing the game on certain holidays will cause the characters to wear holiday-related costumes during the loading screens. On Xmas, it also plays "God Rest Ye Merry Gentlemen" during gameplay.
In The Neverhood, clicking on the burpberry tree three times will lead to a minute long burping cutscene.
In Grand Theft Auto IV, the Statue of Happiness has a giant heart hidden inside of it.
"This grass feels funny. It feels like... pants." - Kirby
Also a song.