Two that spring to mind immediately (and what a lovely thread)...
Aged around 10.. staying with my Dads best friend and thus, his kids - they had ponies, I was the ponyless pony mad kiddo..
We all set off to walk/ride to a local gymkhana, I was trying to lead a stroppy shetland pony AND push my bike, a highly unsuitable racing bike, across the common and rough tracks... it wasn't going well. I managed to get someone else to lead the pony but then got left behind as they all trotted off and I was still dragging the bloody stupid bike, and by now in tears.
Going past a garden that fronted onto the common, I fell over the bike and really scraped my leg, started howling and swearing.. and a face pops over the hedge... a nice lady invited me to dump the bike there, come in and clean up my leg and did I want to join their BBQ and then go to the gymkhana with them a short while later as they (and their kids) were going too.
Did i! Not only did they clean me up, (find out who I was and who I was with.. its a small rural community so everyone knew everyone)... they LENT ME A PONY to ride for the afternoon!
It was like a dream come true out of some pony book... I'd love to say I then entered all the gymkhana events and won them but I didn't... but the look of EVIL on the faces of the people I was supposed to be with was amazing when I did arrive, in style on a lovely smart grey pony instead of dragging a bike!
I had a lovely afternoon and at the end they drove me and my knackered bike back to where I was staying too!
Another 'kindness of strangers' one - aged around 18, I'd been invited by my then boyfriend to come up and stay with him at his shared house in the town he was at Uni in... I turned up at the appointed time (pre mobile phone ownership) and he wasn't home, in fact no one was and it was HEAVING it down, so I was soaked, standing on the doorstep.
I sat there for an hour and then the man who was renovating the house next door came over and said he'd seen me waiting and couldn't watch any longer, did I want to come in, hang up my stuff on the radiators, put on dry stuff and wait there?
Oh yes I did... (and yeah he coulda been a murdering weirdo but he wasnt)...
I hung up my stuff, found some dry stuff right in the bottom of my bag, got changed and then he said 'bugger waiting, lets go to the pub at the end fo the road'.. so I said it was ok not to bother, as I had no money and he said sod that, come on... so we went and he bought me a bowl of soup and a plate of chips... and then gave me the house key so I could let myself in to collect my stuff once my stupid boyfriend reappeared! Then he left me in the warm pub, and went back left a note on the door saying where I was and went on his way.
Never saw him again - collected my stuff later that evening (boyfriend reappeared about 9pm, i would have been waiting SIX HOURS by then!) and put the key through the door as instructed.
What a kind man - never forgotten that!