Mai Valentine
Moderator
In a crossover that makes little sense, if any, Konami has revealed that Metal Gear Solid 3: Snake Eater will feature a game mode called “Monkey & Snake.” In this game mode, Snake will be thrown onto a level in which he must capture monkeys from Sony’s Ape Escape (Sarugetchu in Japan) videogames. At the Tokyo Game Show, Metal Gear Solid creator Hideo Kojima also mentioned that Konami and Sony have reached an agreement that has Solid Snake showing up in the next Ape Escape game. Kojima-san is a fan of Sony’s Ape Escape series.
Source: GamersMark Network (http://www.gamersmark.com/news/2004/09/1/4439/)

That's probably the weirdest crossover I've ever heard of.