My husband and I do these kinds of things all the time. My husband regularly buys the coffee for the person in line behind him. (One time a woman was very suspicious, thought he was going to hit on her!)
I regularly fork over a couple of dollars for the person in front of me at the grocery store. My mother-in-law did this a lot, and I always admired her generosity.
My teenage son is great with computers, and I persuaded him to help out a neighbor's adult son, who was in a low-paying, dead-end job. He had an opportunity at a much better job, but needed to take some kind of online test that was several hours long. His computer was down, and you're only allowed an hour on the computers at the library. My son fixed his computer for free. The man passed the test, got the job, and is in the process of buying a house and moving his wife and 2 kids out of their apartment! I was really proud of my son, and he feels great, knowing that a simple action on his part has made such a huge impact on a family.