Like SantaCon, this event is a rather recent phenomenon in the annals of New York City street parties. The name, inspired by England's Stonehenge monument, was coined in 1996 by Neil deGrasse Tyson, the director of the Hayden Planetarium.
The best vantage points for Manhattanhenge are the eastern ends of Manhattan's broadest cross streets: 14th St., 34th St., 42nd St. and 57th St. The Tudor City bridge spanning 42nd Street is arguably the best of them all because 1) you don't have to dodge traffic for a perfect view of the sunset and 2) the view also includes photogenic city landmarks like the Chrysler building and the lights of Times Square. The only potential bummers are 1) overcast skies and 2) massive crowds on the bridge -- bring a ladder.
Below, some pix grabbed from the interweb; TOP ROW, full sun, BOTTOM ROW, half sun