The Wikipedia entry on “leetspeak” — a simple substitution code that is part of hacker/SMS/IM jargon — is amazingly complete and full of fun facts:
It is widely believed that the expression “kekeke” comes from Korean players of StarCraft. It is an onomonopoetic Korean phrase similar to the English “hahaha”, Spanish “jajaja” or Japanese “huhuhu”, and is meant to express laughter. It is often used in-game as an expression of exhultation or as a form of mockery. Commonly, it is associated with a simple Starcraft tactic that involves massing a large number of units and using them to rush an enemy base before an opponent is sufficiently prepared to defend. This is often called a Zerg Rush, after the Starcraft faction for whom the tactic was created. The phrase “OMG Zerg Rush! kekeke!!” is sometimes used outside of the game to indicate any form of overwhelming or swarming force.
(via Kottke)