Free fiction: Jerome Bixby's "It’s a Good Life": Text & audio from Best Science Fiction Stories & Free SF Reader
Best Science Fiction Stories links this online HTML version at NickelSystems & Free SF Reader has linked to this online MP3 audio version at timetravelershow of this famous story.
Not exactly a great story in my book - I don't like dark pessimistic stories - but it's included in Robert Silverberg's very widely read SF Hall of Fame, Vol 1. Plot is very similar to Alfred Bester's "Star Light, Star Bright", but more creepy.
In both stories, a young boy's wishes come true - in ways that mean horror for adults.