When the extra 3DS circle pad was revealed, it was shown alongside Monster Hunter 3 G. That's not the whole 3DS game that will support the extra slide pad. There are more. Many more.
Besides Monster Hunter 3 G, the following games support the 3DS Expansion Slide Pad:
Capcom's Resident Evil: Revelations, Konami's Metal Gear Solid: Snake Eater 3D, Namco Bandai's Ace Combat 3D Cross Rumble, Tecmo Koei's Dynasty Warriors VS and Square Enix's still undated Kingdom Hearts 3D [Dream Drop Distance] .
The 3DS Expansion Slide Pad comes to Japan this December for ¥1500 ($19).