ENC28J60 Ethernet module
The cheapest but also a very easy way to add ethernet to microcontrollers like Arduino.
This module has an ISP interface for communication.
Head over to this link https://github.com/jcw/ethercard#readme to learn all about the ENC28J60 Ethernet module, download the library and get Arduino examples to use.
Note: it is important to connect this module to the 3.3V power pin and NOT 5V. 5V is to much power for this board and will damage it.
25MHZ crystal oscillator
3.3V pin header power jack
Dimension 56(mm) x 34(mm)