According to retail research firm the NPD Group, the biggest-selling accessory in the United States is not a controller, memory card or chatpad. It's a pre-paid points card for an online gaming service.
Yes, for the months of both March and April, the top-selling accessory was a 1600 point Xbox Live card, the equivalent of USD$20 in cash. As someone who uses a credit card on a weekly basis for online transactions, this seems absurd, but then I've never had my card details stolen, am over the age of 18, have good credit and have never tried to cancel my Xbox Live subscription over the phone.
Considering at least one of those criteria applies to many of you, though, you can see why, despite this being 2010, people still like to do business the old fashioned way.
The fact the cards go on sale all the time might help, too.
NPD: 1600 MS Points card best-selling 'accessory' for two months [Joystiq]