These coupon codes are given out quite often by hosting companys. We give them away with hosting plans at RegisterBird.com also. I am not so sure about the yahoo credit but I know for a fact 98% of the google adwords vouchers are only allowed to be used on new adwords accounts. So your best bet is to give or sell your google voucher to someone who does not currently use Adwords. If you give it to someone who already has an adwords account, the changes are pretty good that the voucher will be rejected if the account is not relatively new. Also note that if someone has already used a voucher, applying a 2nd voucher in most cases will not work either. Like I said, most adwords vouchers out there are for new accounts only unless google gives you a voucher for being a loyal customer or something. I know they also give away vouchers at conventions and such.