Candidate for best blog post of all time? Quickly made a big impact on a real problem in the computing world.

I remember reading this at the time, installing one of the implementations that immediately popped up (SpamAssassin), and finally having my spam separated from my ham.

Edit: I had not considered until now the possibility that the "IKEA effect" might have made me overestimate the quality of filtering because of the effort that I put into training the classifier!