Serial IP (SerialIP.h)

http://playground.arduino.cc/Code/SerialIP -- SerialIP is a port of the uIP TCP/IP stack to the Arduino. It allows a stock Arduino (with no shields) to access the Internet and to run Internet accessible servers. It works by using SLIP to route TCP/IP data over the serial port, just like a dialup Internet connection (only in this case it's the Arduino "dialling up" to your PC, and there are no modems or telephone calls involved.) Because all traffic is routed over the serial port, it means you can get network access without needing the Ethernet shield. However unlike the Ethernet shield which handles the TCP/IP protocol for you, SerialIP implements the entire TCP/IP stack (courtesy of the fine uIP project.) This has a couple of downsides, the main one being size. It takes up around 9kB of flash space and enough RAM that you can easily run out of memory (which normally results in the Arduino rebooting unexpectedly.) However it is possible to work within these limitations, and with no additional purchase beyond the Arduino itself it's certainly a cheap introduction to the world of embedded networking!