Dagen H (The H day) was the day, 3 September 1967, on which traffic in Sweden switched from driving on the left-hand side of the road to the right. The H stands for Högertrafik, the Swedish word for "right-hand traffic".
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proposes a list of about seventy-five(!) left-driving countries and territories.
Also a link to http://en.wikipedia.org/wiki/Dagen_HAnd Dagen H commemorative underwear.