My personal best was Overstock.com.
I bought SMB3 on GBA in October for about $11. They sent me the strat guide, not the game. After speaking with customer service, I was told that I could return it (at their cost) for a full refund, which was fine with me.
But before I ever even got the return label, they sent me a copy of the game. At this point I assumed that, provided I returned the guide, they would just keep my $11 as payment for the game, which was fine by me. Then, about a week later, they refunded my money.
Theoretically, I got the game for free, not considering the hassle of returning the guide and staying in contact with their service dept.