Tell us a story: helping a stranger

Tell us about a time when you helped out a stranger, not for any reward but purely as an act of kindness — just because you happened to be there and it was clear that they needed help.  Where were you?  What happened?  What did you do to help?  How much of your time did it take?  Did your efforts make a difference?  What might have happened if you hadn't stepped in to help?