An implementation of the TCP/IP protocol stack that uses very little resources. Code size is on the order of a few kilobytes and memory usage is on the order of a few hundreds of bytes. http://www.sics.se/~adam/uip/
lwIP is a lightweight implementation of the TCP/IP protocol stack. The focus of the lwIP TCP/IP implementation is to reduce RAM usage. http://www.sics.se/~adam/lwip/
A simple implementation of TCP & FTP, suitable for burning into ROM. It is, in effect, a hack put together in two or three days written in C--in 1986. http://www.unusualresearch.com/tinytcp/tinytcp.htm