This is a newer leaderboard, created in February 2010. It ranks Gamertags based on what it calls a TrueAchievement score. This is similar to Gamerscore, except all achievement points are weighted according to how rare the achievement is (i.e. how difficult it is to obtain). It's a great concept, and you might find your position quite different from what it is on the other leaderboard. This one is open to anyone, so simply join the website and then add yourself to the leaderboard.
Visit the Neowin.net leaderboard at TrueAchievements.com
This is the original leaderboard, created back in October 2006 at the same time this topic was made. It ranks Gamertags based on pure Gamerscore, so the more achievement points you have, the further you will be up the table. The downside is it's limited to only 250 members, so if you want to join you need to send me a PM so I can add you manually the next time I update the list.