Matchbox computers: Small is beautiful (and powerful)
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.
Raspberry Pi was one of the first open-source matchbox computers: inexpensive, tiny and energy-efficient.