Half-Life: Portal is an influential computer game with an intricately self-referential story. The protagonist (and I am eliding wildly and am anyway basing this on Wikipedia) is tricked by GLaDOS, an artificial intelligence into undertaking a series of tests. The protagonist is repeatedly promised cake, as a whimsical sort of reward.
As the game progresses, it becomes clear that GLaDOS has other-than-stated ends. Clearly previous 'players' have been sent through the same tests, and there is graffiti saying 'The Cake is a Lie'. The protagonist has to kill GLaDOS. This has entered some parts of popular culture as a shorthand: you are told by your heartless, manipulative boss that working hard might lead to promotion; you look cynically at your fellow peon and mutter, 'The cake is a lie.'
The game's credits then whisk the viewer through tunnels and etc., and into a central room, with a cake, which is extinguished by a mechanical hand while GLaDOS starts singing:
There are many, many fan versions of this song on YouTube. It's won all kinds of awards and its composer, Jonathan Coulton, played it at a concert for computer game fans because he 'knew it was one of those things that would just make people’s heads explode.'*
Computer game music is very big news, by the way. In a world where pop songs routinely replace the once sweeping scores of movies, computer games are still doing lush, big-style stuff. Japan is where it's at, people. Here's the Royal Stockholm Philharmonic Orchestra playing pieces from the Final Fantasy series.
* This did not happen