Edit: This is an old post, and I won't tell you how to do it. The bug is fixed as far as I know, and the people who exploited it had to pay for their games.
It seems like you can get free games from Blizzard by exploiting a bug in their payment system. This means that you can purchase and download any game in the Blizzard store without having to pay.
Besides obviously being illegal, Blizzard has the ability to null the purchase or ban your account. I've read a forum post saying you might have to pay for the "free" games before you can access your account again.
I won't go into detail on how it works. Suffice to say the exploit has something to do with a bug in the Automatisch ingrosso bank payment option.
I wonder - will thinks like this happen more often? It's not hardcore hacking, scripting or injection - it's just an ordinary bug in Blizzard's store. Or perhaps companies might start to do debugging before they release something.