Ethernut is an Open Source Hardware and Software Project for building Embedded Ethernet Devices:
The hardware design includes a small board, which is equipped with an Atmel ATmega128 CPU and a Realtek RTL8019AS Ethernet Controller. It can be easily expanded with add-on boards attached to its expansion connector.
The well documented software is an Open Source implementation of a Real Time Operating System called Nut/OS and a TCP/IP protocol suite named Nut/Net. Several application examples are provided, including an embedded Webserver, a simple RS232 to TCP/IP gateway and an Internet Radio playing MP3 streams.
A well documented API and more than 100 kByte program space and 20 kByte data memory are available to easily implement custom applications. The source code is written almost only in C and available for download. The licence allows its use in Open and Closed Source Projects without paying royalities, even not in commercial applications. The developer can choose between two different development environments, the freely available GNU AVR Compiler Collection for Linux and Windows and the commercially supported ImageCraft AVR Compiler for Windows.