A big seller in 2011 was the 3DS Mario Kart 7 handle. Despite its somewhat goofy appearance, the accessory sold so much that it was the number one topic of customer phone calls to the Hori sales department during the Christmas season. Read Kotaku's review of the peripheral here.
Outside of the portable console arena, a big seller for the Xbox/PS3 was the arcade stick—namely Hori's Joystick Real Arcade Pro. Fueled by the console versions of big budget fighting games like Super Street Fighter IV and Gundam Extreme VS, the arcade stick was the item of choice for Japanese gamers in 2011.
As for the number one top selling item in 2011, with the release of the Nintendo 3DS and the PS Vita, the top selling item was screen protecting filters. Offered in a variety of types from matte to glossy, privacy filters and scratch resistant, screen protectors are an extremely popular item in Japan, where gamers like to keep the screens of their portable game systems scratch-free to an almost anal level. (I should know… All my systems have screen protectors.)
After a year marred by natural and nuclear disasters, Peripheral makers expect that 2012 should yield better sales and more quality accessories to fit the needs of every gamer out there.