Loads...
I slipped on some ice a few years ago and couldn't get up again. A total stranger came along, got me up, ascertained that I had probably broken my elbow (I had) and went and got his wife to help. Turns out I vaguely knew the wife - she is a colleague, but in quite a large school. She looked after my toddler and baby while DH took me to hospital.
I've had my purse handed in several times when I've lost it, always with the contents intact. I try to write and say thank you when they leave an address.
When DD1 was a few weeks old, my car key snapped in the lock and I was totally stuck. I ran with pushchair to a locksmith's, which was about to shut. They stayed open late to help me - I had to call out the AA to retrieve the missing piece before they could make me a copy. It was a few days before Christmas - aaw. I took the man a hand made Christmas card for that one.
I only do little things really - buy the odd cup of tea or snack for homeless people, pass on tickets, etc. A couple of years ago, a guy was sleeping in the bus shelter near our house one night, and it was snowing. DH got chatting to him, and he was a really nice guy - he almost invited him in, but we had tiny children and he decided you can't be too careful... so he took him duvets and a warm sleeping bag, which were returned in the morning.