You can also use the Ethernet Shield as a client. In other words, you can use it to read websites like a web browser.
Websites have a lot of text both visible and hidden, which makes programming on the client side very tricky. Reading information from websites typically involves parsing a lot of strings. This is maddening, but worth it, if that is what you intend to do.
I was going to write some code to read Twitter messages, but such a code already exists as an example within the Arduino programmer. Instead, I simply modified it slightly to turn on an LED if a special message is read.
To make this work connect the positive lead an LED to pin D2, and the negative lead in series with a 220 ohm resistor to ground.
Don't forget to enter your own IP address into the code below, or it will not work.
Here is the code:
Presumably you are going to want to read something other than the recent post on the RandyMcTester Twitter feed.
To read other Twitter feeds, change the following bit of text:
client.println("GET /1/statuses/user_timeline.xml?screen_name=[NEW TWITTER NAME HERE]