It's unclear who said:

"I think there is a world market for maybe five computers"

but it might have been Thomas Watson in 1943, president of IBM, but it might be a myth.

The thinking was that a number of large mainframey computers would be built and used by the world. That of course, didn't happen, as my watch has more memory than ENIAC did, and there's computers everywhere, but as the web continues to mature, I believe that things will conflate in the next 10-15 years and as more and more companies begin to

outsource storage to services like S3

move servers into Virtual Machines

move all email and apps to Google Apps

All this is building a collective trust with large entities like Amazon, and as prices fall with uptimes rising, more companies will say, "who am I to build a datacenter? I'll just host in one of The Five."

Here's a list of services (by no means exhaustive) from Wikipedia's Utility Computing article, as of today:

Amazon S3 - Bulk storage and bandwidth for static content

Amazon EC2 - Pay by the hour CPU

NearlyFreeSpeech - Pay as you go web hosting for web pages, dynamic content, domains, DNS, etc

Sun Microsystems Sun Grid - Pay by the CPU hour

Strikeiron Web Services Marketplace - Pay per Web API call.

USi an AT&T company - USiPinnacle - Pay-as-you-go enterprise applications

Zimki - J avascript based utility computing system. Pay by bandwidth, storage, and javascript operations.

ElasticLive Utility web hosting service based on Amazon's Elastic Compute Cloud

InsynQ utility computing services

CPAASP online accounting solutions for on-demand enterprises

Distributed Potential Pay-Per-Use Grid Computing Capacity

Dell Inc. have a specific Cloud Computing Solution through their Datacenter Solutions Division

I believe this list will likely turn into The Five Computers:

Google (Apps) Amazon (EC2) Microsoft (Live) Sun (Grid) A network of installed bots on every small computer, possibly built into the OS, to use idle CPU cycles for the collective.

This "5th computer" will be the "remainder" after the first four, but might ultimately become the largest. Perhaps it's a larger number, but surely Amazon will buy Bay at some point, and Google will buy Salesforce.com, so they don't count.

Computing will be moved into the Cloud. It's already happening, we're 20% there. The idea has been around since the beginning, and it will, in my opinion, continue come up until it actually happens and we build Skynet. One of these compute clouds will no doubt end up in orbit.