A GUID is a 128-bit integer that can be used across all computers and networks wherever a
unique identifier is required. Such an identifier has a very low (that's really low!) probability
of being duplicated.
The easiest easy way to get a GUID from the Web is built on top of setgetgo.com.
Getting a GUID
Simply fire a GET request to http://setgetgo.com/guid/get.php, in return you'll get the GUID.