CEO: Eric Schmidt, former CTO of Sun and former CEO of Novell, took the helm in 2001.

How Google got into cloud computing: Google Apps was the company's attempt to branch out beyond the consumer search market and become a player in the enterprise. Google unveiled the enterprise version of Apps in February 2007 in a competitive strike against rival Microsoft, and followed up by releasing App Engine in April 2008.

Who uses the service: Lots of small businesses, enterprises and colleges including Arizona State University and Northwestern University.~~

Company name: GoGrid (a division of ServePath) 

Launched: March 2008 (ServePath was founded in 2001, GoGrid development began in 2006)

Location: San Francisco

Cloud offering: The GoGrid platform offers Web-based storage and the ability to quickly deploy Windows- and Linux-based virtual servers onto the cloud, with preinstalled software including Apache, PHP, Microsoft SQL and MySQL.

Why we're watching it: GoGrid, one of Amazon's chief competitors in the cloud storage and compute markets, distinguishes itself from Amazon in a couple ways. GoGrid offers Windows Server 2008 instances (Amazon offers only Windows Server 2003) and 100% uptime service-level agreements (Amazon offers 99.95% for compute and 99.9% for storage).

CEO: John Keagy, the CEO and founder of ServePath, built and sold several ISPs in the decade prior to starting ServePath.

How GoGrid got its start: Executives at ServePath, a dedicated server hosting company, created GoGrid after deciding that inefficiencies within the standard hosting model could be alleviated with a self-service, pay-as-you-go infrastructure.

Who uses the service: Mostly start-ups, Web 2.0 and SaaS companies, plus a few big names like SAP and Novell who are running pilots or small test projects on the GoGrid service.

Company name: Microsoft 

Founded: 1975

Location: Redmond, Wash.

Cloud offering: Azure, a Windows-as-a-service platform consisting of the operating system and developer services that can be used to build and enhance Web-hosted applications. Azure is in beta until the second half of 2009.

Why we're watching it: Because this is Microsoft's first big foray into the cloud. But for all of Microsoft's might, it is still a new player in the cloud market and has questions to answer. For example, will it be easy to move existing applications onto the Azure platform, and will Microsoft avoid the tendency toward vendor lock-in – which is bad for users but has been tremendously profitable for Microsoft in the world of packaged software.

CEO: Steve Ballmer, appointed CEO in 2000 after 20 years with the company.

How Microsoft got into cloud computing: Microsoft made its name by developing the operating system for home and work computers. But with all forms of applications moving to the Web-hosted model, it's no surprise Microsoft would make Windows available over the cloud. Microsoft also provides a set of business services over the Web, including Exchange, SharePoint, Office Communications Server, CRM and Live Meeting.