This Ethernet Shield which is based on the Wiznet W5100 Ethernet Chip gives you an easy way to get your Device Online. It is directly supported by official Ethernet Library. It adds a micro-SD card slot, which can be used to store files for serving over the network.    It is compatible with the Duemilanove (168 or 328), Uno as well as Mega (1280/2560) and can be accessed using the SD library.
The Wiznet W5100 provides a network (IP) stack capable of both TCP and UDP. It supports up to four simultaneous socket connections. Use the Ethernet library to write sketches which connect to the internet using the shield.

Test Code:
/*  
 * Web Server  
 *  
 * A simple web server that shows the value of the
analog input pins.  
 */  
 #include <Ethernet.h>  
 byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };  
 byte ip[] = { 192, 168, 0, 15 };  
 Server server(80);  
 void setup()  
 {  
 Ethernet.begin(mac, ip);  
 server.begin();  
 }  
 void loop()  
 {  
 Client client = server.available();  
 if (client) {  
 // an http request ends with a blank line  
 boolean current_line_is_blank = true;  
 while (client.connected()) {  
 if (client.available()) {  
 char c = client.read();  
 // if we've gotten to the end of the line (received a
newline  
 // character) and the line is blank, the http request
has ended,  
 // so we can send a reply  
 if (c == 'n' && current_line_is_blank) {  
 // send a standard http response header  
 client.println("HTTP/1.1 200 OK");  
 client.println("Content-Type: text/html");  
 client.println();
// output the value of each analog input pin  
 client.print("welcome to B2CQSHOP");  
 client.println("<br />");  
 client.print
("//*************************************");  
 client.println("<br />");  
 client.print("");  
 client.println("<br />");  
 client.print
("//*************************************");  
 client.println("<br />");  
 for (int i = 0; i < 6; i++) {  
 client.print("analog input ");  
 client.print(i);  
 client.print(" is ");  
 client.print(analogRead(i));  
 client.println("<br />");  
 }  
 break;  
 }  
 if (c == 'n') {  
 // we're starting a new line  
 current_line_is_blank = true;  
 } else if (c != 'r') {  
 // we've gotten a character on the current line  
 current_line_is_blank = false;  
 }  
 }  
 }  
 client.stop();  
 }  
 }