Nowadays computer programs seem to get very huge (exception to that are some embedded software running on tiny microcontrollers). There are still some examples that if you know what you are doing you can do amazing thing with a very little amount of code. For example The International Obfuscated C Code Contest and Assembly computer festival demo competitions have shown that in some cases you can do amazing things with few kilobytes of code or even less.
Toledo 8080 emulator! is a winner of 19th IOCCC. Intel 8080 emulator. 19th IOCCC. Best of Show. page tells that this emulator of the 8080 processor implemented with just 2000 characters of C. That code can even run CP/M operating system on that emulated processor.
Look also 8th IOCCC (1991) entry DDS Basic Interpreter. It is a full basic interpreter (capable of running classic Basic games like Moon Lander) with only 20 lines of code.