The open-source uIP TCP/IP stack provide TCP/IP connectivity to tiny embedded 8-bit microcontrollers, with maintained interoperability and RFC standards compliance.

uIP is an implementation of the TCP/IP protocol stack intended for small 8-bit and 16-bit microcontrollers. It provides the necessary protocols for Internet communication, with a very small code footprint and RAM requirements – the uIP code size is on the order of a few kilobytes and RAM usage is on the order of a few hundred bytes.

uIP is open source software written in the C programming language and the documentation and source code is free to use and distribute for both commercial and non-commercial use as long as proper credit is given (the full BSD-style license is here). It has been ported a wide range of 8-bit microcontrollers and is used in a large number of embedded products and projects (see the Links page for a few examples).

Look at:
http://www.sics.se/~adam/uip/index.html