Poetry Foundation
Poetry Foundation: Baseball Poems
A collection of descriptive and metaphorical poems about baseball, including poems by Mae Swenson, John Updike, and William Carlos Williams, and, of course, "Casey at the Bat" by Ernest Lawrence Thayer.
Other
History Buff: The Story Behind "Casey at the Bat"
This website provides information on the history of Ernest Lawrence Thayer's "Casey at the Bat," tracing the poem's rise from obscurity to "immortal."