Tired of your crappy old Gamercard? Then spruce it up with the Xbox Live Gamercard Generator! The XLGG operates on the same basic principles as MyGamerCard.net, but with some important differences.
Update 1: as you can read in the comments, MyGamerCard.net has grown quite a bit since I last used it. More options and user submitted themes actually put it ahead of XLGG in many respects. Not only that, it's also officially supported by Microsoft. Thanks for setting me straight, everyone.