Go here to visit coupons.com (the main website people know about).
Then there is Red Plum printables here.
Smart Source also has printables available here.
A master list of printable coupons is available here.
A new website I just learned about (thanks Bucktown Bargains) is called Coupons, Inc and is just a huge list of printable bricks coupons (coupons from the coupons.com site). I have only spent a few minutes on it and am in love so far. It is a lot of coupons that are not on the coupons.com website, but once you fill out a form or hit print, are linked back to that website. Go here to visit and print today!