I still do, and that's the truth.
I believed that Santa delivered gifts to my family and I long after most of my peers had given up. I had one other friend my age who still believed. Our logic: We knew our parents couldn't afford all the riches found under the tree every year. My mother helped me along: In my house, Santa (who still takes credit for many presents) has the most magical curly, wavy handwriting.
Although I now acknowledge that, in an objective, factual, scientific sense, a character named Claus is not the one to buy or wrap the presents under the tree, it pains me to deny out loud the woolly-bearded man or his elvish crafts-people. So I don't. If my sister or I want to thank Mom for the hard work she's put into Christmas - and the magic she lends to it - we exclaim, "What a [wonderful/shiny/generous/cool/just-right] gift! Tell Santa I said 'Thank you!'" and give her a big hug.