Library of Congress
Loc: "O Captain! My Captain!"
This site from the Library of Congress provides comments on a poem written by Walt Whitman at the death of Abraham Lincoln. Image of the manuscript available at this Library of Congress site.
Other
Bob's Byway: Glossary of Poetic Terms
Calling itself "unique," Bob's is easy to use, with cross-links throughout, phonetic pronunciation guides when necessary, and many examples and quotations. Click on the letter and scroll for the word.