I don't think I've ever done something particularly noteworthily-awesome for a stranger- though I suspect that at least two strangers would say 'OH GOD I LOVE THIS PERSON'.
I think the best story I have of me being nice to a stranger was me being on a bus with a couple who were arguing about Where the House that 'Interview With The Vampire' was set in was located, and telling them 'I'm so glad I was on this bus 'cause I have Anne Rice's viewpoint on it. You'll wanna get off two stops from here, and walk back to Madrone: the upstairs flat is where she envisioned the interview occurring'.
Best thing a stranger ever did for me was answering my kinda broken-French query about 'how do we get to the Eiffel Tower from here?' by taking us both through the labyrinthine series of passages that go from the Metro to the not-exactly-Metro that goes to the Eiffel Tower, and then simply turning around and walking away and not even answering us shouting 'Thank you!' in French after him.