I'm lucky enough to have a few of these that spring to mind:
When I was pregnant with DD1, I fainted on a busy train into London. A doctor and a midwife happened to be sitting in the carriage and looked after me; several other people sitting nearby checked up on me later in the journey, and brought water and sweets. I was so touched.
Also, when DD1 was 2 and I was very pregnant with DD2, my car skidded on the M6, hit the central reservation and spun across the carriageway into the hard shoulder. It was still raining, I had struggled up this muddy embankment carrying DD1 who was getting upset. I was in shock but a lovely man had seen the crash and stopped, came to check I was ok, brought us a blanket and waited with us till the police arrived. As soon as they did, he quietly left and in all the commotion I never got the chance to properly thank him.
This is the last one: Just a few months ago, I was in a car park in a bad area. I'd been walking with bags full of shopping, buggy etc, DD1 was being a handful, but I'd finally got us all in the car and was about to start the engine when a really scary looking man with a dog on a string came up to my window. My immediate thought was to lock my doors, or that he was going to ask me for money or was pissed - he genuinely did look homeless, & he may have been. I did wind down my window and he was bringing me my wallet which he had seen me drop halfway across the car park. My faith in human nature was well and truly confirmed, and it was a lesson to me not to be so quick to judge 