The Arduino Ethernet Shield allows you to connects Arduino to your local LAN or Internet with Ethernet connection.. Just plug Arduino Ethernet Shield module onto your Arduino board, connect it to your network with an RJ45 cable. The original Arduino Ethernet Shield is based on the Wiznet W5100 Ethernet chip. Arduino IDE comes with example code. Everything else is good in The Arduino Ethernet Shield except the price which is typically in $30-45 range, which is expensive. This is why I have earlier used cheaper alternative Ethernet shield.
When the price gets right, I was ready to go to the original design. Now I found a very cheap Arduino Ethernet Shield: for € 6.12 ($8.15) I can buy Ethernet Shield Module W5100 Micro SD Card Slot For Arduino UNO MEGA. With this price it might not be worth to spend more time in trying to get cheaper alternatives to work.
Ethernet Shield Module W5100 Micro SD Card Slot For Arduino UNO MEGA is directly supported by Arduino official Ethernet Library. It comes with micro-SD card slot. The Wiznet W5100 provides a network (IP) stack capable of both TCP and UDP, supporting up to four simultaneous socket connections.
Ethernet Shield Module W5100 Micro SD Card Slot For Arduino UNO MEGA works well on Arduino UNO with WebServer example on Arduino IDE 1.0.5. Thing worked well as expected. This card seems to be well worth of the price. It had the features I wanted (Ethernet connection) in the price range I liked.
I have not had time to test other applications for this Ethernet shied. Here are some interesting looking things I might plan to try: ArduinoCommander controlling, Pin Control Over the Internet – Arduino + Ethernet and RESTduino – Arduino hacking for the REST of us.