Criss Crowe has published a cool script to display the web site ID and description.
With IIS 6 the Web Site ID is a randomly generated number for each site that is created other than the Default Web Site which has an Web Site ID of 1)
Knowing which web site these being to is a problem as it requires you to manually look at each web site. The following script will allow you to output the ID and name.
More at source: Displaying the Web Site ID and the Description.