Every Thursday evening (not at any given time), Old Navy updates this coupon website. You have to click around and look for the coupons. It's like a treasure hunt. For instance here is where one of the coupons were hidden this week (so that you get an idea):
There was a mannequin in the picture holding a carton of movie-popcorn. If you clicked on the popcorn box, popcorn started to pop out. After clicking multiple times, the coupon for $10 off of $50 popped up.
They always have a few of the $75 off of $100 (though I think those are near impossible to get). I dunno - has anyone gotten one yet???
Otherwise, they usually have a bunch of others: 30% off, $5 off $25, $10 off $50, etc. They are great coupons, but you can only print out one and they can't be shared. You kind of have to be online right when they make the change, because apparently this is NOT a best-kept secret. A lot of people know about it. There are even MULTIPLE web forums dedicated to people giving hints on where to find the coupons, and which ones they got, etc...