What is your favorite random act of kindness?

I recently was on the receiving end of a free latte at the Panera Bread drive through, courtesy of the car ahead of me (who also paid for the two cars behind me). I have done this same thing in the Starbucks drive through, but being on the receiving end has made me want to remember to do it more frequently because it really DOES brighten a person's day. If anything it reminds us to pay it forward with simple gestures like holding doors open or saying something kind to a stranger, or giving that Mom at target with the toddler having a tantrum a nod and smile.
What are your favorite examples of random acts of kindness?