Small Internet Protocol Stack using a standard serial port.
Years ago I was fascinated by images of the World's Smallest Webserver next to a matchbox.
Below, an mbed-LPC11U24 board running PPP-Blinky serves an HTTP 1.1 webpage to a browser.
This modern ARM-based version of a tiny webserver uses the USB port for both power and communication.
Edit: Someone pointed out that this is smaller: http://hackaday.com/2016/09/03/software-usb-on-the-esp8266/
For more information and/or the code, click on PPP-Blinky below.
If you want to find out more about mbed-enabled hardware go here https://developer.mbed.org/platforms/
If you want to find out more about the mbed online compiler go here https://developer.mbed.org/compiler/