One great source of high-value coupons I've found over the last several months is Vocalpoint. Vocalpoint is an online community that posts surveys, product reviews, and articles on various topics. Go here to sign up with them and indicate that you want to receive special offers, they will send you all kinds of fun stuff. Most recently, I have gotten an envelope full of $1 coupons for Rice Krispies, plus a coupon for a completely free box, and a $2.50 coupon and several $1.50 coupons for the new Bounce dryer bar. Other past promotions that I've gotten included Kashi waffles (a coupon for a free box, plus several $1.50 coupons), $4 coupons for Venus Embrace razors (these coupons ended up making these free after Register Rewards at Walgreens), Frosted Mini Wheats, Crest toothpaste, Kashi cereal bars, Del Monte Fruit Naturals, and probably several others.

As far as I can tell, I have never gotten spam or unsolicited mail from them - just great coupons and sometimes free samples.