If you use a particular brand, check their website for coupons or specials. Some offer a print on-demand option, but some still have you fill out your contact information and mail you a coupon. These sites may have print limits, but go ahead and try as many times as you can!
Yoplait Yogurt – Special Promotion to Try a FREE cup! (Valid as of Jan 4, 2010)
Dog Food – Ceasar.com
Cat Food – Whiskas.com
Excedrin.com – I just got $3.50 off any Excedrin PM product in the mail.
Zyrtec (allergy med) – Coupons for $2 or $6 off
Stayfree – Feminine Products
Betty Crocker – Covers a variety of products, you must sign up, but they give you a preview of the coupons (with $ amounts).