The latest entry comes from HP, which on Wednesday introduced a line of x86 servers designed for what HP calls "extreme scale-out" environments. The HP ProLiant SL servers have a layout that lets fans run at lower speeds, and omit features that HP says often aren't required by large Internet companies, such as redundant power supplies and advanced management software.

Slideshow: 5 Tools to Prevent Energy Waste in the Data Center

But HP is not the pioneer in this scale-out server market, with companies such as IBM, Sun, Dell, Super Micro, and Rackable offering products of their own.

Each vendor has its own approach, but in general these are "systems optimized for large homogenous application scale-out deployments, an application that spans 1,000 servers," says Forrester Research analyst James Staten. "These are typically things delivered by Web services, or cloud services as the new buzz term goes."

One key factor is recognizing the server is no longer the point of reliability and availability, Staten says. A software layer -- such as a virtualization platform -- is needed to ensure the application survives the failure of any particular node, and "the server just needs to be as cheap and efficient as possible," he says.

HP claims its new products will let customers "cut acquisition costs by 10% and power draw by 28%, while doubling their compute density." These claims would be hard to verify, because HP has not revealed pricing of the servers.

But customers buying thousands of servers can reasonably expect up-front and operational costs to decline by tens of percentage points, says Illuminata analyst Jonathan Eunice. Besides improvements in power and cooling efficiency, new servers offered by HP and rivals "strip down traditional high-availability features and management features to optimize the cost and the supply chain for massive build-out," Eunice says.

This approach is similar to the thinking behind blade servers, which eliminate various components to save space and power costs. But these new scale-out servers may be most similar to what Google has built in its own server farms. Google executives recently explained in a blog post that "we strip down our servers to the bare essentials, so that we're not paying for components that we don't need. For example, we produce servers without video graphics chips that aren't needed in this environment."