A celebration
http://www.sylviaplath.info/biography.html
A comprehensive site on the author, with a detailed biography, photographs, and information on her poetry and prose works.
Modern American Poetry: Sylvia Plath
http://www.english.illinois.edu/maps/poets/m_r/plath/plath.htm
The University of Illinois's companion website, with copious criticism on the most famous poems, a 1962 interview with the poet, and more.
Poetry: Out Loud
http://www.bbc.co.uk/arts/poetry/outloud/plath.shtml
Sylvia Plath reads "Lady Lazarus" for the BBC.
Sylvia Plath
http://www.poets.org/poet.php/prmPID/11
Poets.org presents: a biography; related articles on confessional poetry and Robert Lowell, among others; information on Ariel; and external links.
Plath Profiles
http://www.iun.edu/~nwadmin/plath/
Indiana University Northwest's academic journal Plath Profiles. A multitude of scholarly articles on everything to do with the writer.