Soliloquy
Site of local theatre bookstore. Can search by author and title. Includes many plays not listed in Amazon or Barnes and Noble.

Inter-Play
An index by author and title to plays in collections, anthologies, and periodicals. Includes many plays not cited in the standard printed play indexes.

Mr. William Shakespeare and the Internet
This site attempts to be a comprehensive annotated guide to the scholarly Shakespeare resources available on the Internet.

playdatabase.com
Search this database for plays and monologues.

Small-Cast One-Act Guide Online
The name is self-explanatory. Can search by author, title, and cast size/gender.