Microsoft are the ones for giving out free coupons, although I've not had much success with advertising I've had 4 or 5 coupons. I suggest if you signup to google and microsoft but do it when you transaction card is going out of date. When it does go out of date they will contact you to tell you and if your lucky they'll give you a coupon to intice you back.
The was also a few Facebook coupons but they might now be out of date.
But I think this myspace coupon might still work;
MySpace Advertising $75 free code | Online Auction Owners