Yale University
Yale New Haven Teachers Institute: Musical Theater
Introductory site focusing on American musical theatre and Broadway. Shows how these parts can be taught in the classroom. Useful for teachers and students working with music theater.
Wikimedia
Wikipedia: Operetta
This Wikipedia site offers excellent information on the artform of the operetta, including its history, important contributors, and hyperlinked terms.
Other
The Aria Database
The searchable Aria Database features over 1,200 arias from 170 operas by 60 composers. It includes lyrics, translations, synopses of operas, and MIDI files.
El Poder de la Palabra (The Power of the Word)
E Pd Lp: Pablo Sorozabal
This page has a brief bibliography of the Spanish composer and author of some of the operettas Pablo Sorozabal.