The Contiki operating system is impressive: a 42 K program that runs happily in 64 K of random access memory (RAM) on an ancient Commodore 64 microcomputer. It harks back to 1980, when I was 10 and we had our first home computer, a TRS-80 Model I.
The Model I had 4 K of RAM and no disk. To load a program, you either typed it in from a book or used a special cassette player (using regular audio tapes) to play ugly sounds into the computer, like a very slow modem. A typical application might take 15 or 20 minutes to load, so sometimes typing was faster. Around the same time, there were more advanced machines such as the Apple II Plus and, later, the Commodore 64 itself.
Contiki runs on that old machine, but includes a frightening array of modern features: multitasking, graphical windows, Internet access, Web browsing, and even a small Web server -- all in 42 K. It's almost magic, since few people seem to be able to write even a text editor that doesn't require a few megabytes of memory and disk space anymore. By comparison, my current Mac has 416 MB of RAM, more than 6,500 times as much as Contiki requires.
The Mac has a Web server, browsers, and windows too, by the way. I'm not too far behind.