Poets.org is a great archive of information on America's poets. Frost's entry has his biography, links to his poems, and critical essays about him.
Frost taught at the Bread Loaf School of English at Middlebury College on and off for 42 years, between 1921 and 1963. The school has a great online archive about the poet, including a wide selection of primary documents.
The people who produce the journal Poetry have a great page about Robert Frost, with an insightful critical biography and links to his work. Their suggested bibliography is especially worth a look.
Stone House, the home in Shaftsbury, Vermont, where Frost lived from 1920 to 1929, is now a museum. Its website is a useful trove of Frost information, with pictures of all of Frost's homes and a free library of Frost-related articles. It also offers a special tutorial section for students.
This site accompanied a 1996 exhibit on Frost at the Clifton Waller Barrett Library of American Literature. The online offerings aren't extensive, but they contain some rare and interesting pieces of Frost memorabilia, such as images of several of his family's Christmas cards.
The Library of Congress has a mini-site focusing on Frost's famous performance at John F. Kennedy's inauguration. Frost's instant substitution of another poem when he was unable to read the one he brought is one of the greatest saves in literary history—not bad for an 86-year-old man.