First: this is NOT from inside Blizzard
Second: I know this isn't exactly an exploit, but I know this is one of the more popular forums and I really wish to share it.
I work at a local Best Buy, it's a pretty cool place to work. Now, whilst I was busy shelving our pre-paid WoW cards, one of my superiors saw me and commented "Man, I love those things, it's cheaper for us to buy them then to pay Blizzard directly!", sure enough, my employee discount drops the 2-month card (normally $30) down to $24. If you happen to work at a location that carries these cards (or know someone who does), you may be able to get a small discount.
But wait, there's more! I'd also like to let you know that we have our best price for starting WoW right now (sorry if I sound like an advertisement).
World of Warcraft : $17.99 (normally 19.99)
Burning Crusade: $26.99 (normally 29.99)
plus you get a FREE $20 GIFT CARD for purchasing them at the same time.
If you are looking into getting a new set of CD-Keys or recruiting a friend, it's only going to cost you 26.98! I know it's not exactly free, but you will still save some twocents, and it's 100% legal.
Just thought I'd share.