You could sum up most of the history of computing in one word: smaller. Each successive generation of computing devices has been tinier, more energy-efficient and more powerful than the previous one. Now we've reached a point where an entire PC can be crammed into a space not much larger than a matchbox or a stick of gum.
This new wave of "matchbox computers" (also known as "thumb PCs") has ushered in not just new form factors but new kinds of applications. Hobbyists have flocked to these tiny systems, attracted by their size, low cost and inherent hackability.
Many of these devices are built either on open, freely reproducible architectures, or with well-documented components. Their appeal isn't just limited to the U.S., either: the Raspberry Pi was developed in the UK, and the Odroid is a South Korean product.
Even big PC makers are now getting into the game: Dell's Project Ophelia, an Android device on a stick, will run Android-based apps when plugged into a display's HDMI port. It debuted at CES this year; the developer's version should ship this month and the consumer version in July. It is expect to have a $100 price tag.
What led to matchbox devices?
Matchbox computing owes its existence to the confluence of several different trends:
Linux, GNU and FOSS. The Linux kernel and the GNU toolchain, products of the culture of free/open-source software (FOSS), have been used as the common substrate for any number of hardware designs. These range from set-top boxes and networking gear (with a little help from derivative projects like BusyBox) to Android-powered devices. Android itself, too, has been put to use in the same way.
Consequently, many matchbox devices are powered either by a Linux distribution of some kind, or by a stock edition of Android. Linux alone is most widely used on devices where the user interface is either minimal -- no more than a command-line interface is needed, if even that -- or where one needs to be custom-built for the device. Android, on the other hand, is useful for system-on-chip devices with built-in multimedia -- graphics, sound, HDMI-out, etc.
System-on-chip (SoC) devices. A good deal of the most recent engineering for SoCs has been for the smartphone and ultrabook market, with smartphones showing off SoC designs at their most compact, power-efficient and feature-laden. And since SoCs consist of very few components by design, that makes them easier to build a device around. Wireless networking is also included by default in most SoC designs, which makes them even more compact because a network port doesn't have to be included.