I was at the receiving end of a Random Act a few months ago. I was trying to buy an Evening Standard (when you still had to pay for it) on my way home from work and only had a £10 note. The bloke (very apologetically) said he couldn't change it. A young woman behind me bought two copies and gave me one of them. Made me smile all the way home.
I was walking down the road with my son a month or so back with a very large parcel, something I'd sold on ebay and was taking to the post office. A royal mail delivery guy who happened to be walking past stopped me, said he was just delivering a parcel to a house up the road, but that if I waited by his van he'd sort out the paperwork and take it away. Bless him!