(1) Single dads. Yes, they are both the mother and the father of the children they raise.
(2) Widowers who lost the mother of their children, and children/adults who've lost their mother. Share a happy memory you have of her with the survivors and offer a hug.
(3) Any woman whose child passed away (either as a child or as an adult) especially needs to be acknowledged. Trust me, she hasn't forgotten giving birth nor her parenting memories. A lot of people are afraid that saying something may make her depressed. In reality, not saying anything will cause the greatest saddest and make her feel as if nobody remembers.
Make more than just your mom feel loved today. Happy Mother's Day!