"Scattered throughout tropical seas are coral reefs that started when a ship sank and sea creatures made it their home. Then the predators of those creatures started hanging out, and their predators, all the way up the food chain. Eventually, if the ocean climate was right, a coral reef would appear, much larger than the wrecked ship that started it all."

I like this metaphor because it easily explains the eco-system of applications that spring up around a given product. Windows has many apps out there to keep it secure and stable, the iPod has spawned countless other companies to support it, and the eco-system of Amazon.com ranges from IMDB to cloud computing to affiliate marketing to warehousing services for everyone.