Pardon us if we hadn't noticed this before, but in a recent Xbox365 article, a forumgoer noted this site, Gamertag Database, which at the time of this writing lists 8,701 Xbox Live gamertags worldwide. The site isn't connected to Microsoft in any way; users can volunteer to register their own gamertags in the database, along with their location and email address, and users are encouraged to rate other gamers who use the service (think of it as a Heavenly Hosts list for XBL). There's a FAQ on the site if you have any questions. Looks interesting.