At dawn on the winter solstice, the passage and chamber of the megalithic passage tomb at Newgrange are illuminated for 17 minutes by a shaft of sunlight entering through the roofbox above the entrance. The builders of Newgrange achieved this precise alignment over 5,000 years ago, 1,000 years before Stonehenge. You can watch the sunrise illumination on a live webcast between 08:30 and 09:30 UTC on Sunday, December 21st.
The Thirteen Towers of Chankillo in Peru may be the Western Hemisphere's oldest known full-service solar observatory, showing evidence of early, sophisticated Sun cults, according to archaeoastronomy professor Clive Ruggles. The 2,300-year-old complex featured 13 towers running north to south along a ridge and spread across 980 feet to form a toothed horizon that spans the solar arc. Last year, another ancient observatory was discovered in Peru by Robert Benfer. The Temple of the Fox is 4,200 years old, making it 1,900 years older than the Chankillo site, but wasn't a complete calendar.
Ancient observatories from space Satellite images of Angkor Wat, Chichen Itza, Chaco Canyon, Stonehenge, Teotihuacan, and others. The observers, observed. High res images available.
The Chaco Culture National Historical Park (flash) encompasses a whole canyon's worth of buildings that appear to be designed to elaborately showcase the movement of the sun and the moon. And the website for the park is pretty well done. Also see the PBS-supported documentary called "The Mystery of Chaco Canyon" from the Solstice Project and a previous Metafilter discussion of archaeoastronomy.