To find a particular poem:
LION (Literature Online)
Representative Poetry Online. Searchable database from the University of Toronto includes 3,162 English poems by 500 poets from Caedmon, in the Old English period, to the work of living poets today.
Bartleby Verse. This site includes, among other materials, a searchable collection of anthologies of American and English poetry. Includes "thousands of poems by hundreds of authors"
Columbia Granger's Index to Poetry in Anthologies
Widener RR 3062.20
Master Index to Poetry
Widener RR 3062.20.3
Columbia Granger's Index to Poetry in Collected and Selected Works
Widener RR 3062.20.5