Old Navy has a Facebook app where you play their little game to win a
printable in-store coupon.
Link: http://www.facebook.com/oldnavy?v=app_10442206389
$10/$50 is the very least you should be able to get. Refresh until you win
one you want.
I managed a $15/$75 but there are $50/$100 available.
Valid through 9/9/10.
Sorry if this is old info but I couldn't turn up anything in search.