For 100 coins, you can get Dr. Mario Online Rx (WiiWare) or the Virtual Console version of Kirby's Dream Land (3DS). For 150 Coins each, you can get The Legend of Zelda: Majora's Mask (Wii VC) or Dr. Mario Express (3DS). Note that Dr. Mario Express is actually a DSiWare game, but this offer can only be redeemed for a 3DS download code.
If you're interested in turning your coins (which you got from buying games) into one of these games, you have until January 31. Nintendo will cycle in more games after that point.