Yahoo serves cocktails from its back-end (Photo: aoife mac/Flickr)

Google, Amazon, and Yahoo began life as websites, but they’ve evolved into something more. Now, they share their underlying infrastructure with the outside world, letting other businesses take advantage of the complex hardware and software systems they’ve erected over the years.

Google lets outsiders run applications atop its infrastructure via Google App Engine. Amazon offers its Elastic Compute Cloud (EC2). And sometime next year, Yahoo will launch “Manhattan” — an online service where anyone can build and host applications usi...