The news that US telecommunications provider AT&T has joined the rapidly growing ranks of cloud computing providers reinforces the argument that the latest IT outsourcing model is well on its way to becoming a classic disruptive technology.

By enabling datacenter operators to "publish" computing resources -- such as servers, storage, and network connectivity -- cloud computing provides a pay-by-consumption scalable service that's usually free of long-term contracts and is typically application- and OS-independent. The approach also eliminates the need to install any on-site hardware or software.

Currently dominated by Amazon.com and several small startups, cloud computing is increasingly attracting the interest of industry giants, including Google, IBM, and now AT&T. "Everyone and their dog will be in cloud computing next year," predicts Rebecca Wettemann, vice president of research at Nucleus Research, a technology research firm.

Yet James Staten, an infrastructure and operations analyst at Forrester Research, warns that prospective adopters need to tread carefully in a market that he describes as both immature and evolving. Staten notes that service offerings and service levels vary widely between cloud vendors. "Shop around," he advises. "We're already seeing big differences in cloud offerings."

To help cut through the confusion, here's a rundown some major cloud providers -- both current and planned -- all offering resources that go beyond basic services such as SaaS (software as a service) applications and Web hosting:

3Tera: appliance-driven virtual servers

3Tera's AppLogic is a grid engine that has evolved over time into a full-fledged cloud computing environment. The company says its offering is designed to enable datacenters to replace expensive and hard-to-integrate IT infrastructure -- such as firewalls, load balancers, servers, and SANs -- with virtual appliances. Each appliance runs in its own virtual environment.

AppLogic combines servers into a scalable grid that's managed as a single system via a browser or secure shell. According to 3Tera, datacenters can add or remove servers on the fly, monitor hardware, manage user credentials, reboot servers, install software, build virtual appliances, back up the system, repair damaged storage volumes, inspect logs, and perform every other management tasks from a single point of control, all while the system is running.

Amazon.com: As-you-need-them basic IT resources

Amazon was an early cloud computing proponent, and the company now has one of the market's longest menu of services. Amazon's core cloud offering, the Elastic Compute Cloud (EC2), provides a virtualized cloud infrastructure that's designed to provide scalable compute, storage, and communication facilities.

Amazon's cloud computing arsenal also includes the Simple Storage Service (S3), a persistent storage system, as well as the Simple Database (SimpleDB), which provides a remotely accessible database, and the Simple Queuing Service (SQS), a message queue service that's also an agent for tying together distributed applications created by the EC2, S3, and SimpleDB combo.