Friedrich Hölderlin
Biographical essay on the German lyric poet, with links to related topics.
http://en.wikipedia.org/wiki/Friedrich_H%C3%B6lderlin

Encyclopædia Britannica: Holderlin, Johann Christian Friedrich
Biographical entry in the 11th edition (1911). Some scanner errors.
http://encyclopedia.jrank.org/HIG_HOR/HOLDERLIN_JOHANN_CHRISTIAN_FRIE.html

Friedrich Hölderlin's Life, Poetry, and Madness
Excerpts from Wilhelm Waiblinger's essay, translated by Scott J. Thompson, about the poet's delusional state in later years, so distressing to those who knew him.
http://www.wbenjamin.org/holderlin.html

Internet Encyclopedia of Philosophy: Hölderlin, Johann Christian Friedrich
Examines the poet's role in the development of German Idealism.
http://www.iep.utm.edu/h/holderli.htm

Johann Christian Friedrich Hölderlin
Short illustrated biography, and two poems in English translation.
http://andrejkoymasky.com/liv/fam/bioh3/hold2.html