There are different purposes and respective different constructions of ethernet modules.
Popular types that release the circuit of complex TCP/IP protocol activities are
- ethernet to serial bridges, e.g. Lantronics XPort devices
- ethernet interface chips with hardware TCP/IP stack, e.g. Wiznet W5100 **broken link removed**
Same point as before, depends on the usage of ethernet in your application. In my opinion, Wiznet (preferably operated through SPI interface) would be a good choice. You can also refer to the Microchip Applications based on ENC28J60, they are ready to use, but there is large part of ethernet protocol located in software compared to Wiznet chips.
If this is your first time implementing an Ethernet interface, I would recommend use a module with an Embedded TCP/IP Stack rather a device like the ENC28J60 which requires a significantly larger coding overhead to implement the required TCP/IP Stack.
The Wiznet W5100 Module that FvM recommended would be an excellent choice.
Many of the modules with Embedded TCP/IP Stacks only require you to implement a conventional asynchronous or synchronous serial interface to connect with the module and issue the appropriate API commands via this serial interface to control the module and the resulting Ethernet Interface.