British Library
British Library: Discovering Literature: Piers Plowman
Read an overview of the poem "Piers Plowman" by William Langland (possibly) and view additional resources such as articles and collection items.
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.