Once upon a time, if you wanted a PC, your options were limited to large machines. Because interchangeable parts were so important to build the PC as a commodity, manufacturers were reluctant to design anything smaller because the form factor would change and be incompatible with everything on the market. Very small Unix machines were on the market, such as the Sun SPARCstation IPC and IPX, but they were expensive.
This is changing. Increased demand from consumers, small dimensional standards taken largely from notebook computers, and special applications have created a market for a new class of machines. Form factors such as Mini-ITX are now on the market. They aren't the fastest computers around, and they aren't overly expandable, but they have big advantages. They are small, quiet, use less power, generate less heat, and are astonishingly inexpensive. Linux has made a contribution to the increasing popularity of these machines. Whereas a computer with relatively low processor power stood no chance against a market using Microsoft's resource-hungry operating systems, the ever-increasing population of Linux users does not care. Manufacturers know this and make sure that their products support Linux.
The current small machines are hybrids. The motherboards can still fit into standard cases, use normal-size memory, and often have a single PCI slot. They accept standard PC components, even the power supply. However, because many of the components are self-contained and generate little heat (including power supplies), it has become fashionable to custom-build smaller enclosures. These machines really shine when placed in special small cases that will not accept larger components.
One common example of this is the CD-ROM drive. On a regular desktop computer, these devices are gigantic because they fit into a half-height 5 1/4-inch drive bay. To make them fit in a smaller machine, an easy substitution for the normal size CD-ROM drive is a notebook CD-ROM drive. Like the embedded processors found on many small motherboards, a notebook CD-ROM drive may not be as quick as its larger cousin, but it is a fraction of the size.