What is the kindest thing a stranger has done for you?

Returned my cellphone when I left it on the bumper of my car in the middle of Queens...

I was queuing at the pub and the person in front of me asked me what I like. I told him what beer I like and he got it for me. The best thing is that was also my bday. I really appreciated his generosity.

Someone gave me their seat on a busy tube carriage once :)

Traveling on the Interstate in VA late at night, needed gas badly so pulled off at next exit to go to a station.. naturally the credit card reader was offline, and I had no cash. Couldn't find another gas station around when we saw a policeman talking to a woman outside of a closed store - pulled up and asked if they knew where another station was but there wasn't one close by. The woman hands me $5 and tells me to go to the original station, buy gas, and then drive up a couple of exits to where another gas station was. She refused to let me pay her back (somehow, told her I would mail her a check :) and the Officer just looked at us both like we were crazy people ;D