As usual, the Poetry Foundation offers a good, modest-sized biography, as well as a selection of classic poems.
The National Library of Ireland presents this fantastically rich online exhibit, complete with images, poems, and letters.
Check out this BBC History site for some historical context on the Irish fight for independence.
The Internet is so cool. You don't have to go to Yale to check out this lecture from a modern poetry course about Yeats, made available for free online.
The only known recording of Yeats reading his poetry is this short, early work.
A young Yeats looking mighty bookish. By this time, he had probably already proposed to Maud Gonne at least once.
Can you see the "sternness"?
A painting that depicts the destruction of the ancient city.
Read some of Yeats and Gonne's letters to each other, courtesy of Google Books' preview.
Helen Vendler is one of the best critics of Yeats's poetry. Among other things, she discusses how "No Second Troy" helped Yeats refine his mastery of the sonnet form, taking his influence from Shakespeare. Check out the Google Books preview here.
Richard Ellmann is your go-to man for biographies about early-20th century Irish writers: he also covered James Joyce and Oscar Wilde. In this biography of Yeats, you can learn (much) more about Yeats's tortured love for Maud Gonne.